[clang] [clang] Enable --print-supported-extensions for all targets (PR #66586)

[Balint Cristian via cfe-commits]

cbalint13 wrote:

@DavidSpickett ,

This PR list all flags for a specific target.

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Additionally, beyond this PR:

* We can limit to list flags only for a desired e.g. -mcpu=XYZ, e.g. the user can query like:
```./bin/clang -target x86_64-linux-gnu -mcpu=sandybridge --print-supported-extensions```

* The cost for this would be simply iterating through ```MCInfo->checkFeatures()``` for a specific ```-mcpu```.

Let me know if it would be interesting future, I'll be happy doing it in a subsecvent PR.

https://github.com/llvm/llvm-project/pull/66586