[ADT] function_ref's constructor is unavailable if the argument is not callable.

This allows overload sets containing function_ref arguments to work correctly
Otherwise they're ambiguous as anything "could be" converted to a function_ref.

This matches proposed std::function_ref, absl::function_ref, etc.

Differential Revision: https://reviews.llvm.org/D88901

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -579,9 +579,7 @@ public:
   /// \returns true if an error occurred.
   Error parseBitcodeInto(
       Module *M, bool ShouldLazyLoadMetadata = false, bool IsImporting = false,
-      DataLayoutCallbackTy DataLayoutCallback = [](std::string) {
-        return None;
-      });
+      DataLayoutCallbackTy DataLayoutCallback = [](StringRef) { return None; });
 
   static uint64_t decodeSignRotatedValue(uint64_t V);