Enable index store based on Clang feature detection

[neonichu]

Today, the index store is only enabled on Darwin by default and needs a manual opt-in on other platforms. We can instead switch this to enabling it based on whether the used clang supports -index-store-path.

rdar://117744039

[neonichu]

@swift-ci please test

[neonichu]

Not really sure if we should land this in the current form, it does make builds on macOS slightly slower for no good reason (since we're always doing the feature detection through shelling out to clang).

I see two paths forward:

• we find a better way to do the feature detection that is acceptable on macOS as well
• we still enable unconditionally on macOS, but that makes the test useless (which is fine in principle, we can delete it)

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test windows

[bnbarham]

Is there any way to disable the index if it isn't wanted?

[neonichu]

Is there any way to disable the index if it isn't wanted?

Not right now, do you should think that should be possible?

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test windows

[neonichu]

@swift-ci please test windows

[neonichu]

@swift-ci please test macOS

[bnbarham]

Not right now, do you should think that should be possible?

Yes. There's likely an issue for it... somewhere.

[neonichu]

Actually, we do have --disable-index-store

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test windows