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/Target/ARM/AsmParser/ARMAsmParser.cpp b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
index c011c79..78e033f 100644
--- a/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
+++ b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
@@ -5425,7 +5425,7 @@
       return false;
     }
     // w/ a ':' after the '#', it's just like a plain ':'.
-    // FALLTHROUGH
+    LLVM_FALLTHROUGH;
   }
   case AsmToken::Colon: {
     S = Parser.getTok().getLoc();