Re: Windows default locale vs initdb

I clicked "Trigger" to get a Mingw test run of this, and it failed[1].
I see why: our function win32_langinfo() believes that it shouldn't
call GetLocaleInfoEx() on non-MSVC compilers, so we see 'initdb:
error: could not find suitable encoding for locale "en-US"'. I think
it has fallback code that parses the ".1252" or whatever on the end of
the name, but "en-US" hasn't got one. I don't know the first thing
about Mingw but it looks like a declaration for that function arrived
6 years ago[2], and deleting the "#if defined(_MSC_VER)" fixes the
problem and the tests pass[3]. As far as I know, we don't support any
Mingw but the very latest: it's not a target with real users who have
version requirements, it's just a developer [in]convenience, so if it
passes on CI and whatever MSYS version "fairywren" runs in the build
farm right now, that should be enough.

I could just do that in this patch, but I suppose that also means that
someone needs to go through pg_locale.c and other places that test
_MSC_VER not because they actually care about the compiler but because
they want to detect some crusty old Mingw version, and see what else
can be deleted as a result, possibly including a lot of fallback code.
It feels like a separate cleanup for a separate patch.

[1] https://cirrus-ci.com/task/5301814774464512
[2] https://github.com/mirror/mingw-w64/blame/eff726c461e09f35eeaed125a3570fa5f807f02b/mingw-w64-tools/widl/include/winnls.h#L931
[3] https://cirrus-ci.com/task/6558569718349824