[clang] [clang][Driver][Darwin] Optionally use xcselect to find macOS SDK (PR #119670)

[Carlo Cabrera via cfe-commits]

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@@ -1,3 +1,6 @@
+// XFAIL: xcselect
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carlocab wrote:

In theory, yes. In practice, it looks to me like all these failing test cases[^1] have deployment targets that pre-date macOS 10.14, so it seems very likely that anyone who can enable `CLANG_USE_XCSELECT` will probably see these tests fail as soon as they do.

But, point taken. Do you have an alternative approach in mind?

I'm wondering if it's worth reviving the `CLANG_NO_XCSELECT` environment variable and then just setting them in these tests (and slowly getting rid of instances of `CLANG_NO_XCSELECT` as we find better fixes).

[^1]: I haven't checked each individually, so I'm not sure yet.

https://github.com/llvm/llvm-project/pull/119670