Added LLVM_FALLTHROUGH to address warning: this statement may fall through. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304647 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Rewrite/HTMLRewrite.cpp

@@ -409,6 +409,7 @@ void html::SyntaxHighlight(Rewriter &R, FileID FID, const Preprocessor &PP) {
       ++TokOffs;
       --TokLen;
       // FALL THROUGH to chop the 8
+      LLVM_FALLTHROUGH;
     case tok::wide_string_literal:
     case tok::utf16_string_literal:
     case tok::utf32_string_literal: