[clang] Use cmake to find perl executable (PR #91275)

[Matthias Braun via cfe-commits]

MatzeB wrote:

I checked that:
- scan-build tests run fine by default on my machine.
- scan-build tests are skipped when using `cmake -DCMAKE_DISBALE_FIND_PACKAGE_Perl=ON ...`.

https://github.com/llvm/llvm-project/pull/91275