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/Parse/ParseCXXInlineMethods.cpp b/clang/lib/Parse/ParseCXXInlineMethods.cpp
index 5898120c..848a09b 100644
--- a/clang/lib/Parse/ParseCXXInlineMethods.cpp
+++ b/clang/lib/Parse/ParseCXXInlineMethods.cpp
@@ -728,7 +728,7 @@
     case tok::semi:
       if (StopAtSemi)
         return false;
-      // FALL THROUGH.
+      LLVM_FALLTHROUGH;
     default:
       // consume this token.
       Toks.push_back(Tok);
@@ -1108,13 +1108,13 @@
         goto consume_token;
       if (AngleCount) --AngleCount;
       if (KnownTemplateCount) --KnownTemplateCount;
-      // Fall through.
+      LLVM_FALLTHROUGH;
     case tok::greatergreater:
       if (!getLangOpts().CPlusPlus11)
         goto consume_token;
       if (AngleCount) --AngleCount;
       if (KnownTemplateCount) --KnownTemplateCount;
-      // Fall through.
+      LLVM_FALLTHROUGH;
     case tok::greater:
       if (AngleCount) --AngleCount;
       if (KnownTemplateCount) --KnownTemplateCount;
@@ -1219,7 +1219,7 @@
     case tok::semi:
       if (CIK == CIK_DefaultInitializer)
         return true; // End of the default initializer.
-      // FALL THROUGH.
+      LLVM_FALLTHROUGH;
     default:
     consume_token:
       Toks.push_back(Tok);