[TableGen] Use std::move properly (NFC) (#166104)

This patch removes const to allow std::move a couple of lines below to
perform a move operation as intended.

Identified with performance-move-const.

Author: kazutakahirata

llvm/utils/TableGen/Basic/ARMTargetDefEmitter.cpp

@@ -220,7 +220,7 @@ static void emitARMTargetDef(const RecordKeeper &RK, raw_ostream &OS) {
                           ProfileLower + "'");
 
     // Name of the object in C++
-    const std::string CppSpelling = ArchInfoName(Major, Minor, ProfileUpper);
+    std::string CppSpelling = ArchInfoName(Major, Minor, ProfileUpper);
     OS << "inline constexpr ArchInfo " << CppSpelling << " = {\n";
     CppSpellings.push_back(std::move(CppSpelling));