[lit] Stub out lit feature detection on non-Apple platforms

[yln]

Quick fix for an oversight on my part that slipped through CI in my previous PR (#23455): the TSan runtime now has a link-time dependency on libdispatch (unfortunately).
This temporarily disables the following two tests on Linux to unbreak CI build.

Swift(linux-x86_64) :: Sanitizers/witness_table_lookup.swift
Swift(linux-x86_64) :: Sanitizers/tsan.swift

The above two tests are also the only 2 TSan (executable) tests that are run on Linux. Once I fix the TODO mentioned in this PR, I will go through all REQUIRES: tsan_runtime occurences and try to enable them by using libdispatch/foundation instead of objc_interop.

[yln]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 50c82a5ac99cb5466c487f0d25db7b487d341f47

[yln]

@swift-ci Please test Linux

[yln]

@swift-ci Please smoke test osx

[yln]

@jrose-apple Are you okay with the updated patch? This should unblock #23508

[jrose-apple]

I'm a little iffy on whether or not this will work because I'm not sure if we're consistent about "dispatch" meaning "Swift Dispatch" or "C Dispatch", but maybe it won't matter. It's okay for unblocking your other work, though, so go for it.

[yln]

I'm not sure if we're consistent about "dispatch" meaning "Swift Dispatch" or "C Dispatch", but maybe it won't matter.

Good point. Here (the lit feature) means "Swift Dispatch", i.e., everything required for import Dispatch. "C Dispatch" is required for -sanitize=thread when linking. "Swift Dispatch" implies "C Dispatch" (but not the other way around). Ideally, these would all be separate things.

[compnerd]

This broke the test suite on Windows! https://dev.azure.com/compnerd/windows-swift/_build/results?buildId=823

[jrose-apple]

Can you, uh, tell us how it broke the test suite on Windows?

[compnerd]

#23585 should repair the issue as a quick repair, looking into what it would take to properly detect this in #23588

[compnerd]

Can you, uh, tell us how it broke the test suite on Windows?

lit.py: C:\agent\_work\1\s\llvm\utils\lit\lit\TestingConfig.py:102: fatal: unable to parse config file 'C:/agent/_work/1/s/swift\\test\\lit.cfg', traceback: Traceback (most recent call last):
  File "C:\agent\_work\1\s\llvm\utils\lit\lit\TestingConfig.py", line 89, in load_from_path
    exec(compile(data, path, 'exec'), cfg_globals, None)
  File "C:/agent/_work/1/s/swift\test\lit.cfg", line 1503, in <module>
    config.available_features.remove('tsan_runtime')
KeyError: 'tsan_runtime'