devel/llvm18: misc improvements

Fix worse runtime performance on Zen CPU when optimizing for Zen. [0]

Install all standard heaaders with clang. Historically we've been
unable to build FreeBSD with the full set due to conflicts and/or
missing features between the compiler provided headers and
FreeBSD's headers. I've verified that I can build world and kernel on
the main, stable/14, and stable/13 branches for amd64 so let's give it
another try in broader testing. [1]

PR: 278908 [0], 274542 [1]