[libcxx] [test] Prepare the ctime.timespec test for mingw CI env upgrades

[mstorsjo]

Summary

The test is currently marked XFAIL for mingw environments, but in
the future, it will start passing. (I'll update this with a reference
to the mingw-w64 commit once I've pushed it.)

The CI environment will probably be upgraded to a state where it is
passing only after 14.x is branched in the llvm-project monorepo.

If we'd just go from having an XFAIL to no marking at all (when CI is
passing), we'd have to update both main and 14.x branches in sync
exactly when the CI runners are updated to a newer version.

Instead, mark the test as temporarily unsupported (so it doesn't
cause failed builds when the CI environment is updated); after the
CI environments are upgraded to such a state, we can remove the
UNSUPPORTED marking to start requiring it to pass on the main branch,
without needing to synchronize that change to anything else.

[ldionne]

Out of curiosity, are you coordinating on the update to the testing environment?

Please make a note to come back and remove this :-)

[mstorsjo]

In D116132#3205682, @ldionne wrote:

Out of curiosity, are you coordinating on the update to the testing environment?

Please make a note to come back and remove this :-)

Yeah, I'll coordinate to update the CI images once 14.0.0 is out for both versions (clang-cl and llvm-mingw) that are packaged in the CI image, and remove this afterwards. (There's another XFAIL with a clang-13.0.0 version marking that we should be able to remove after that too.)

libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp

 // std::timespec and std::timespec_get
 
 // UNSUPPORTED: c++03, c++11, c++14
 // XFAIL: LIBCXX-AIX-FIXME
 // ::timespec_get is provided by the C library, but it's marked as
 // unavailable until macOS 10.15
 // XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12|13|14}}
 
-// MinGW lacks timespec_get.
-// XFAIL: target={{.+}}-windows-gnu
+// MinGW used to lack timespec_get, but has got it in newer versions.
+// Mark the test as unsupported for a transition period, to avoid spurious
+// failures when upgrading the precommit testing environment. After
+// the testing environment is upgraded, we could remove the marking altogether.
+// UNSUPPORTED: target={{.+}}-windows-gnu
 
 #include <ctime>
 #include <type_traits>
 
 #ifndef TIME_UTC
 #error TIME_UTC not defined
 #endif
 
 std::timespec tmspec = {};
 static_assert(std::is_same<decltype(std::timespec_get(&tmspec, 0)), int>::value, "");