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

[Fangrui Song via cfe-commits]

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@@ -1,5 +1,4 @@
-// FIXME: Actually, "perl".
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MaskRay wrote:

Instead of `REQUIRES: perl` in (almost?) every scan-build/ test, update
`clang/test/Analysis/scan-build/lit.local.cfg` instead?

There are some `if ... not config.xxx: config.unsupported = True` examples in other `lit.local.cfg` files.

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