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/Sema/SemaTemplateVariadic.cpp b/clang/lib/Sema/SemaTemplateVariadic.cpp
index 7d1d4c5..3338cec 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -976,6 +976,7 @@
                    PDiag(diag::note_parameter_pack_here));
       ParameterPack = Corrected.getCorrectionDecl();
     }
+    break;
 
   case LookupResult::FoundOverloaded:
   case LookupResult::FoundUnresolvedValue: