Enable more tests on llvm-mingw and some fixes

[alvinhochun]

Changes include:

• Since a partial windowsnumerics.impl.h header has been added to mingw-w64 recently, we can enable the tests that depend on it. If it is not available in the current mingw-w64 toolchain, there is an option to download a copy of it from the mingw-w64 git repo to allow these tests to run.
• Enable some test cases of test_cpp20 and test_old for llvm-mingw. test_old in particular depends on windowsnumerics.impl.h)
• Re-enable test code for Clang fixed by Remove low-level coroutine suspension notifications #1228.
• Enable some test cases for llvm-mingw that crashes only when built using Clang with MSVC ABI.
• Make some tests relying on error messages pass on non-English Windows systems.
• Use lowercase filename when including Windows headers, which is needed for cross-building from Linux with mingw-w64.

[sylveon]

Since this lambda has no captures, it can be converted to a normal function with __stdcall.

[alvinhochun]

That can work though it's not as clean as it currently is. However, the test that actually requires this code cannot be built due to missing __uuidof(IContextCallback) (which has been fixed on mingw-w64 upstream, but the current llvm-mingw 15 toolchain doesn't have the fix), so there is no harm just leaving this out for now.