Handling of va_list on Cygwin environment must be matched to normal
Windows environment.
The existing test `test/CodeGen/ms_abi.c` seems relevant, but it
contains `__attribute__((sysv_abi))`, which is not supported on Cygwin.
The new test is based on the `__attribute__((ms_abi))` portion of that
test.
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Co-authored-by: jeremyd2019 <github@jdrake.com>
