[llvm] [NFC][LLVM][ADT] Change a few `const StringRef &` arguments to value (PR #163994)

[Jakub Kuderski via llvm-commits]

================
@@ -21,6 +21,7 @@
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/STLForwardCompat.h"
 #include "llvm/ADT/STLFunctionalExtras.h"
+#include "llvm/ADT/StringRef.h"
----------------
kuhar wrote:

Can we avoid this include by sticking to references? STLExtras is very widely used, so I'd like to keep the set of includes minimal if possible

https://github.com/llvm/llvm-project/pull/163994