[clang] [libcxx] [Cygwin][MinGW] Internal class in explicitly-instantiation-declarated template should be instantiated (PR #140145)

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jeremyd2019 wrote:

trying to use this libc++ ci is less than helpful: the jobs keep failing for no apparent reason (cancelled? docker not running?), and whatever seems to be supposed to rerun the failed jobs is not all that reliable about doing so.  Hopefully my llvm-mingw fork will show this technique works, and we can be reassured that being in an ifdef that isn't true for any other platform shouldn't break any other platform either.

https://github.com/llvm/llvm-project/pull/140145