Remove the dependency on Objective C++ in iOS builds.

252ce9c5 introduced the use of
NSTemporaryDirectory() on iOS, which requires Core Foundation, and
Objective C++.

This CL replaces NSTemporaryDirectory() with an equivalent solution
(according to Apple's documentation at [1]) available to C/C++ code.
Avoiding Objective C++ and Core Foundation makes it easier to integrate
googletest in projects that can't use the supplied Bazel build files.

[1] https://developer.apple.com/library/archive/documentation/Security/Conceptual/SecureCodingGuide/Articles/RaceConditions.html#//apple_ref/doc/uid/TP40002585-SW10

PiperOrigin-RevId: 375474990