[AST] Fix an unused-function warning (NFC)

/llvm-project/clang/lib/AST/ByteCode/InterpBuiltin.cpp:26:13:
error: unused function 'isNoopBuiltin' [-Werror,-Wunused-function]
static bool isNoopBuiltin(unsigned ID) {
            ^
1 error generated.

Author: DamonFool

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -23,7 +23,7 @@
 namespace clang {
 namespace interp {
 
-static bool isNoopBuiltin(unsigned ID) {
+LLVM_ATTRIBUTE_UNUSED static bool isNoopBuiltin(unsigned ID) {
   switch (ID) {
   case Builtin::BIas_const:
   case Builtin::BIforward: