Adjust getMCSubtargetInfo signature for LLVM 23+

[DKLoehr]

A recent LLVM PR changed the signature of getMCSubtargetInfo to return a reference instead of a pointer. This adjusts uses of the function in compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp to account for the different signature.

[rustbot]

r? @cuviper

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[cuviper]

Maybe it would be better to have the old code form a reference too? e.g.

Suggested change

	#if LLVM_VERSION_GE(23, 0)
	const MCSubtargetInfo &MCInfo = Target->getMCSubtargetInfo();
	return MCInfo.checkFeatures(std::string("+") + Feature);
	#else
	const MCSubtargetInfo *MCInfo = Target->getMCSubtargetInfo();
	return MCInfo->checkFeatures(std::string("+") + Feature);
	#endif
	#if LLVM_VERSION_GE(23, 0)
	const MCSubtargetInfo &MCInfo = Target->getMCSubtargetInfo();
	#else
	const MCSubtargetInfo &MCInfo = *Target->getMCSubtargetInfo();
	#endif
	return MCInfo.checkFeatures(std::string("+") + Feature);

This way we don't duplicate the "interesting" code in each case.

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