[ADT] Remove a meaningless std::move (NFC) (llvm#156138)

std::move on StringRef is not useful because it's copied anyway.

Author: kazutakahirata

llvm/include/llvm/ADT/StringMap.h

@@ -264,7 +264,7 @@ class LLVM_ALLOCATORHOLDER_EMPTYBASE StringMap
   /// at - Return the entry for the specified key, or abort if no such
   /// entry exists.
   const ValueTy &at(StringRef Val) const {
-    auto Iter = this->find(std::move(Val));
+    auto Iter = this->find(Val);
     assert(Iter != this->end() && "StringMap::at failed due to a missing key");
     return Iter->second;
   }