Replace "fallthrough" comments with LLVM_FALLTHROUGH

This is a mechanical change of comments in switches like fallthrough,
fall-through, or fall-thru to use the LLVM_FALLTHROUGH macro instead.

llvm-svn: 278902
diff --git a/llvm/tools/dsymutil/DwarfLinker.cpp b/llvm/tools/dsymutil/DwarfLinker.cpp
index d0ef7ab..cb8bb7e 100644
--- a/llvm/tools/dsymutil/DwarfLinker.cpp
+++ b/llvm/tools/dsymutil/DwarfLinker.cpp
@@ -1565,7 +1565,7 @@
         !DIE->getAttributeValueAsUnsignedConstant(&U.getOrigUnit(),
                                                   dwarf::DW_AT_external, 0))
       return PointerIntPair<DeclContext *, 1>(nullptr);
-  // Fallthrough
+    LLVM_FALLTHROUGH;
   case dwarf::DW_TAG_member:
   case dwarf::DW_TAG_namespace:
   case dwarf::DW_TAG_structure_type: