[libc++][test] Fix infinite loop when mkstemp fails

[rprichard]

Summary

mkstemp is guaranteed to make at least TMP_MAX attempts to create the
random file, and if it can't, it fails with EEXIST. get_temp_file_name
shouldn't call mkstemp again if it fails with anything other than
EEXIST. A single mkstemp call seems sufficient.

On Android, I've seen mkstemp fail with:

• EROFS (because cwd wasn't set to a writable filesystem)
• EACCES (because cwd pointed to a dir owned by root, but the test program was running as the shell user instead)

Previously, get_temp_file_name would run forever in these situations.

See D4962 and "llvm-svn: 229035"

[ldionne]

LGTM, but please rebase onto main to trigger CI and get a clean run.

[rprichard]

Rebase.

libcxx/test/support/platform_support.h

while (true) {
             _close(fd);
             return Name;
         }
         if (errno == EEXIST)
             continue;
         abort();
     }
 #else
-    std::string Name;
-    int FD = -1;
-    do {
-        Name = "libcxx.XXXXXX";
-        FD = mkstemp(&Name[0]);
-        if (FD == -1 && errno == EINVAL) {
-            perror("mkstemp");
-            abort();
-        }
-    } while (FD == -1);
+    std::string Name = "libcxx.XXXXXX";
+    int FD = mkstemp(&Name[0]);
+    if (FD == -1) {
     close(FD);
     return Name;
 #endif
 }
 
 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
 #ifdef _LIBCPP_HAS_OPEN_WITH_WCHAR
 inline