[libcxx] [test] Simplify get_temp_file_name() for mingw

[mstorsjo]

Summary

Use the same codepaths as for MSVC. Mingw-w64 does have the _mktemp_s
function; on Vista and newer, msvcrt.dll does contain the function,
which ends up called. (Same thing in the UCRT.) In older versions of
msvcrt.dll (older than what libc++ supports), mingw-w64 provides a
fallback implementation.

This effectively reverts 23323e25f896cf44e6d4519ef38f066e45fe408f (and
d07e5c23b40078dcae13f76b091c9e18763ae44a). That commit tried to fix
unspecified MinGW build breakage. The commit added calls to GetTempPath
and GetTempFileName, but those functions weren't declared unless windows.h
was included implicitly via some other header.

[curdeius]

I let @EricWF have a look at it as he was the author of this MinGW specific code. No particular remarks from my side anyway.

[mstorsjo]

In D98526#2623062, @curdeius wrote:

I let @EricWF have a look at it as he was the author of this MinGW specific code. No particular remarks from my side anyway.

@curdeius What would you think of first landing D97456 (to unbreak things), and then rebasing this on top of that (to clean things up more properly), with this one waiting a bit more on feedback from @EricWF?

[curdeius]

That sounds good for me. Actually I had an impression that D97456 already landed.

[mstorsjo]

In D98526#2626390, @curdeius wrote:

That sounds good for me. Actually I had an impression that D97456 already landed.

Well I was just about to land it last week, when I realized that this approach might be a better fix, but I'll land the other one for now.

[mstorsjo]

Rebased after landing D97456

[mstorsjo]

Rebased; I noticed that mingw configurations could have frequent stray failures in tests that are fixed by this, so it'd be good to get it landed.

libcxx/test/support/platform_support.h

 #endif
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <codecvt>
 #include <locale>
 #include <string>
 #if defined(_WIN32)
-#   define WIN32_LEAN_AND_MEAN // Reduce overhead of including windows.h
 #   include <io.h> // _mktemp_s
-#   include <windows.h> // MAX_PATH, GetTempPath, GetTempFileName
 #else
 #   include <unistd.h> // close
 #endif
 
 #if defined(_NEWLIB_VERSION) && defined(__STRICT_ANSI__)
 // Newlib provides this, but in the header it's under __STRICT_ANSI__
 extern "C" {
   int mkstemp(char*);
 }
 #endif
 
 inline
 std::string get_temp_file_name()
 {
-#if defined(__MINGW32__)
-    char Path[MAX_PATH + 1];
-    char FN[MAX_PATH + 1];
-    do { } while (0 == GetTempPath(MAX_PATH+1, Path));
-    do { } while (0 == GetTempFileName(Path, "libcxx", 0, FN));
-    return FN;
-#elif defined(_WIN32)
+#if defined(_WIN32)
     char Name[] = "libcxx.XXXXXX";
     if (_mktemp_s(Name, sizeof(Name)) != 0) abort();
     return Name;
 #else
     std::string Name;
     int FD = -1;
     do {
         Name = "libcxx.XXXXXX";