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/lib/Option/OptTable.cpp b/llvm/lib/Option/OptTable.cpp
index 9260d79..7eafb00 100644
--- a/llvm/lib/Option/OptTable.cpp
+++ b/llvm/lib/Option/OptTable.cpp
@@ -317,7 +317,7 @@
   case Option::SeparateClass: case Option::JoinedOrSeparateClass:
   case Option::RemainingArgsClass: case Option::RemainingArgsJoinedClass:
     Name += ' ';
-    // FALLTHROUGH
+    LLVM_FALLTHROUGH;
   case Option::JoinedClass: case Option::CommaJoinedClass:
   case Option::JoinedAndSeparateClass:
     if (const char *MetaVarName = Opts.getOptionMetaVar(Id))