[NFC][clang-tidy] Disable test for `bugprone-unsafe-functions` for PlayStation

As discussed in [D91000](http://reviews.llvm.org/D91000) with @dyung, the
PlayStation-specific targets are using some custom standard library for
which the current written tests are not appropriate. Even though the
test code defines the `__STDC_LIB_EXT1__` and `__STDC_WANT_LIB_EXT1__`
macros and expected *Annex K.* support, the actual Clang
parser/preprocessor will report these macros as not existing, and thus
fail the tests.

The check reports the **non**-Annex K. functions as suggestions, such as
`fgets()` instead of `gets_s()` to replace `gets()`, so some safe
library suggestions are still there.

This patch is primarily done to unblock the relevant buildbot
[`llvm-clang-x86_64-sie-ubuntu-fast`](http://lab.llvm.org/buildbot/#/builders/139).

This commit partially reverts ed740e741ec22f9aaea09bfc0b87d0801a7c492f,
as the changes to the "caching logic" was not fixing anything.

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