target-cpu=native should also check all the target-features #54688

Because LLVM doesn't know what target-cpu=native is, we currently convert this to an actual CPU family name, relying on the llvm::sys::getHostCPUName function. So if the current CPU's family is Haswell, we convert target-cpu=native to target-cpu=haswell.

As @EFanZh noticed here and here, some Intel Pentiums belong to the Haswell microarch, but they lack AVX.

Here are the flags Clang generates for -march=native:

$ clang -E - -march=native -###
-target-cpu skylake -target-feature +sse2 -target-feature +cx16 -target-feature +sahf
...
-target-feature -avx512ifma -target-feature -avx512dq

Not only does it set the right target-cpu for optimization purposes, it actually manually enables / disables all the features available / not available on the host. This ensures no matter what the microarch is, only available features will be used.

We should probably use the llvm::sys::getHostCPUFeatures to manually get a list of supported target features. This is what Clang does.

I think this issue will supersede both #38218 and #48464. If we can fix this, we will never generate code with unavailable target features.

Reproducing these errors is pretty hard, unless you have a machine with a Haswell Intel Pentium.
I think it might be possible to reproduce this in QEMU with -cpu Haswell,-avx