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

This reverts commit 4cae6228d1.

Breaks GCC build:
http://lab.llvm.org:8011/#/builders/8/builds/33/steps/6/logs/stdio

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

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