[libc++] Adjust tests using ext/* headers that undefine __DEPRECATED

[libc++] Adjust tests using ext/* headers that undefine __DEPRECATED

Several tests undefined __DEPRECATED to avoid warnings as they're
testing the deprecated ext/hash_map. A better way to do this is to use
-Wno-deprecated so it isn't defined in the first place. This prevents
these tests from failing when we give a warning when undefining the
__DEPRECATED macro, as D144654 will do.

For the generated tests however just remove the testing of these
header files, so we don't disable the warning when testing the other
header files.

Differential Revision: https://reviews.llvm.org/D145691