Fix clang -Wimplicit-fallthrough warnings across llvm, NFC

This patch should not introduce any behavior changes. It consists of
mostly one of two changes:
1. Replacing fall through comments with the LLVM_FALLTHROUGH macro
2. Inserting 'break' before falling through into a case block consisting
   of only 'break'.

We were already using this warning with GCC, but its warning behaves
slightly differently. In this patch, the following differences are
relevant:
1. GCC recognizes comments that say "fall through" as annotations, clang
   doesn't
2. GCC doesn't warn on "case N: foo(); default: break;", clang does
3. GCC doesn't warn when the case contains a switch, but falls through
   the outer case.

I will enable the warning separately in a follow-up patch so that it can
be cleanly reverted if necessary.

Reviewers: alexfh, rsmith, lattner, rtrieu, EricWF, bollu

Differential Revision: https://reviews.llvm.org/D53950

llvm-svn: 345882
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index bf53e25..2c7a3d7 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -6867,7 +6867,7 @@
   case NEON::BI__builtin_neon_vcvth_f16_u32:
   case NEON::BI__builtin_neon_vcvth_f16_u64:
     usgn = true;
-    // FALL THROUGH
+    LLVM_FALLTHROUGH;
   case NEON::BI__builtin_neon_vcvth_f16_s16:
   case NEON::BI__builtin_neon_vcvth_f16_s32:
   case NEON::BI__builtin_neon_vcvth_f16_s64: {
@@ -6887,7 +6887,7 @@
   }
   case NEON::BI__builtin_neon_vcvth_u16_f16:
     usgn = true;
-    // FALL THROUGH
+    LLVM_FALLTHROUGH;
   case NEON::BI__builtin_neon_vcvth_s16_f16: {
     Ops.push_back(EmitScalarExpr(E->getArg(0)));
     Ops[0] = Builder.CreateBitCast(Ops[0], HalfTy);
@@ -6897,7 +6897,7 @@
   }
   case NEON::BI__builtin_neon_vcvth_u32_f16:
     usgn = true;
-    // FALL THROUGH
+    LLVM_FALLTHROUGH;
   case NEON::BI__builtin_neon_vcvth_s32_f16: {
     Ops.push_back(EmitScalarExpr(E->getArg(0)));
     Ops[0] = Builder.CreateBitCast(Ops[0], HalfTy);
@@ -6907,7 +6907,7 @@
   }
   case NEON::BI__builtin_neon_vcvth_u64_f16:
     usgn = true;
-    // FALL THROUGH
+    LLVM_FALLTHROUGH;
   case NEON::BI__builtin_neon_vcvth_s64_f16: {
     Ops.push_back(EmitScalarExpr(E->getArg(0)));
     Ops[0] = Builder.CreateBitCast(Ops[0], HalfTy);
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 0b9a0a4..7b024a0 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -680,7 +680,7 @@
       case LangOptions::SOB_Undefined:
         if (!CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow))
           return Builder.CreateNSWMul(Ops.LHS, Ops.RHS, "mul");
-        // Fall through.
+        LLVM_FALLTHROUGH;
       case LangOptions::SOB_Trapping:
         if (CanElideOverflowCheck(CGF.getContext(), Ops))
           return Builder.CreateNSWMul(Ops.LHS, Ops.RHS, "mul");
@@ -2305,7 +2305,7 @@
   case LangOptions::SOB_Undefined:
     if (!CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow))
       return Builder.CreateNSWAdd(InVal, Amount, Name);
-    // Fall through.
+    LLVM_FALLTHROUGH;
   case LangOptions::SOB_Trapping:
     if (!E->canOverflow())
       return Builder.CreateNSWAdd(InVal, Amount, Name);
@@ -3311,7 +3311,7 @@
     case LangOptions::SOB_Undefined:
       if (!CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow))
         return Builder.CreateNSWAdd(op.LHS, op.RHS, "add");
-      // Fall through.
+      LLVM_FALLTHROUGH;
     case LangOptions::SOB_Trapping:
       if (CanElideOverflowCheck(CGF.getContext(), op))
         return Builder.CreateNSWAdd(op.LHS, op.RHS, "add");
@@ -3346,7 +3346,7 @@
       case LangOptions::SOB_Undefined:
         if (!CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow))
           return Builder.CreateNSWSub(op.LHS, op.RHS, "sub");
-        // Fall through.
+        LLVM_FALLTHROUGH;
       case LangOptions::SOB_Trapping:
         if (CanElideOverflowCheck(CGF.getContext(), op))
           return Builder.CreateNSWSub(op.LHS, op.RHS, "sub");
diff --git a/clang/lib/CodeGen/CGObjCRuntime.cpp b/clang/lib/CodeGen/CGObjCRuntime.cpp
index 8390bca..4b6f24a 100644
--- a/clang/lib/CodeGen/CGObjCRuntime.cpp
+++ b/clang/lib/CodeGen/CGObjCRuntime.cpp
@@ -296,7 +296,7 @@
   switch (paramDecl->getType().getQualifiers().getObjCLifetime()) {
   case Qualifiers::OCL_Strong:
     exn = CGF.EmitARCRetainNonBlock(exn);
-    // fallthrough
+    LLVM_FALLTHROUGH;
 
   case Qualifiers::OCL_None:
   case Qualifiers::OCL_ExplicitNone:
diff --git a/clang/lib/CodeGen/ItaniumCXXABI.cpp b/clang/lib/CodeGen/ItaniumCXXABI.cpp
index b67a64b..e755321 100644
--- a/clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ b/clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -3088,7 +3088,7 @@
     }
 
     assert(isa<ObjCInterfaceType>(Ty));
-    // Fall through.
+    LLVM_FALLTHROUGH;
 
   case Type::ObjCInterface:
     if (cast<ObjCInterfaceType>(Ty)->getDecl()->getSuperClass()) {
@@ -4025,7 +4025,7 @@
       switch (CatchType.getQualifiers().getObjCLifetime()) {
       case Qualifiers::OCL_Strong:
         CastExn = CGF.EmitARCRetainNonBlock(CastExn);
-        // fallthrough
+        LLVM_FALLTHROUGH;
 
       case Qualifiers::OCL_None:
       case Qualifiers::OCL_ExplicitNone:
diff --git a/clang/lib/CodeGen/VarBypassDetector.cpp b/clang/lib/CodeGen/VarBypassDetector.cpp
index 2f8a591..859cdd4 100644
--- a/clang/lib/CodeGen/VarBypassDetector.cpp
+++ b/clang/lib/CodeGen/VarBypassDetector.cpp
@@ -78,7 +78,7 @@
         return false;
       ++StmtsToSkip;
     }
-  // Fall through
+    LLVM_FALLTHROUGH;
 
   case Stmt::GotoStmtClass:
     FromScopes.push_back({S, ParentScope});