Target features need to be propagated to LLVM

[smeenai]

See https://godbolt.org/z/3n85TK4ab for an example. CodeGen adds -target-features to LLVM function attributes to propagate -march. CIRGen needs to match this so that we can e.g. use feature-specific intrinsics end-to-end.

[bcardosolopes]

@seven-mile from your target triple explorations, do you have a hint on what are we missing or whether we can map this as part of MLIR datalayout?

[seven-mile]

@bcardosolopes Clang TargetInfo and Flang treat them parallel with target triple, which I believe is the right abstraction. It sounds reasonable to also have cir.target_features cir.tune_cpu, cir.target_cpu ... as optional StringAttr.

clangir/flang/include/flang/Optimizer/CodeGen/CGPasses.td

Lines 72 to 80 in 41078e9

	let options = [
	Option<"forcedTargetTriple", "target", "std::string", /*default=*/"",
	"Override module's target triple.">,
	Option<"forcedTargetCPU", "target-cpu", "std::string", /*default=*/"",
	"Override module's target CPU.">,
	Option<"forcedTuneCPU", "tune-cpu", "std::string", /*default=*/"",
	"Override module's tune CPU.">,
	Option<"forcedTargetFeatures", "target-features", "std::string",
	/*default=*/"", "Override module's target features.">,

[smeenai]

Note that CodeGen adds these as per-function attributes (presumably for LTO purposes), so we'll want to follow-suit.