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

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jeremyd2019 wrote:

I'm running just a libc++ test (with the toolchain built from the previous run, so it shouldn't take 7 hours this time) in https://github.com/jeremyd2019/llvm-mingw/actions/runs/15087868464

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