[Support] Drop unnecessary std::move in JSON.h (NFC) (#166027)

fixUTF8 takes StringRef, so we do not need std::move here.

Identified with performance-move-const-arg.

Author: kazutakahirata

llvm/include/llvm/Support/JSON.h

@@ -318,7 +318,7 @@ class Value {
   Value(std::string V) : Type(T_String) {
     if (LLVM_UNLIKELY(!isUTF8(V))) {
       assert(false && "Invalid UTF-8 in value used as JSON");
-      V = fixUTF8(std::move(V));
+      V = fixUTF8(V);
     }
     create<std::string>(std::move(V));
   }
@@ -591,7 +591,7 @@ class ObjectKey {
   ObjectKey(std::string S) : Owned(new std::string(std::move(S))) {
     if (LLVM_UNLIKELY(!isUTF8(*Owned))) {
       assert(false && "Invalid UTF-8 in value used as JSON");
-      *Owned = fixUTF8(std::move(*Owned));
+      *Owned = fixUTF8(*Owned);
     }
     Data = *Owned;
   }