[libc++] Tidy up how %T and %t are created during configuration checks

Instead of having ad-hoc cleanup in various places, handle all creation
and removal of temporary files and directories inside _makeConfigTest.

As a fly-by, also remove testPrefix since we don't keep any source file
around anymore. Setting a prefix for the files is hence not useful anymore.

Differential Revision: https://reviews.llvm.org/D114390