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

[via cfe-commits]

jeremyd2019 wrote:

maybe try fixing the formatting complaint and see if the libc++ tests will go further?  (have you tested that with the patched clang?  this CI uses a pre-existing clang binary, not the one built in another workflow)

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