[CodeGen] Dubious temporary expression constructing `mc::RegisterMCTargetOptionsFlags`

[chenm16]

llvm::mc::RegisterMCTargetOptionsFlags says

llvm-project/llvm/include/llvm/MC/MCTargetOptionsCommandFlags.h

Lines 66 to 70 in 74eba97

	/// Create this object with static storage to register mc-related command
	/// line options.
	struct RegisterMCTargetOptionsFlags {
	RegisterMCTargetOptionsFlags();
	};

but in

llvm-project/llvm/lib/CodeGen/CommandFlags.cpp

Line 528 in 7573ee1

mc::RegisterMCTargetOptionsFlags();

it is created as a temporary.