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/ParseTentative.cpp b/clang/lib/Parse/ParseTentative.cpp
index 349434f..de39e06 100644
--- a/clang/lib/Parse/ParseTentative.cpp
+++ b/clang/lib/Parse/ParseTentative.cpp
@@ -159,7 +159,7 @@
       ConsumeToken();
       break;
     }
-    // Fall through.
+    LLVM_FALLTHROUGH;
   case tok::kw_typeof:
   case tok::kw___attribute:
   case tok::kw___underlying_type: {
@@ -219,7 +219,7 @@
 
   case tok::annot_cxxscope:
     ConsumeAnnotationToken();
-    // Fall through.
+    LLVM_FALLTHROUGH;
   default:
     ConsumeAnyToken();
 
@@ -1352,8 +1352,8 @@
     if (Next.isOneOf(tok::kw_new,       // ::new
                      tok::kw_delete))   // ::delete
       return TPResult::False;
+    LLVM_FALLTHROUGH;
   }
-    // Fall through.
   case tok::kw___super:
   case tok::kw_decltype:
     // Annotate typenames and C++ scope specifiers.  If we get one, just