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

llvm-svn: 304650
diff --git a/clang/lib/AST/ASTStructuralEquivalence.cpp b/clang/lib/AST/ASTStructuralEquivalence.cpp
index 9376ee1..eff1aa5 100644
--- a/clang/lib/AST/ASTStructuralEquivalence.cpp
+++ b/clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -424,6 +424,7 @@
       return false;
 
     // Fall through to check the bits common with FunctionNoProtoType.
+    LLVM_FALLTHROUGH;
   }
 
   case Type::FunctionNoProto: {