devel/llvm15: Update to 15.0.1

Of note: -Wimplicit-function-declaration and -Wimplicit-int have been
downgraded to warn by default rather than error to allow more time for
configure scripts to be updated to avoid spurious errors. They are
expected to switch back to error in Clang 16.

For a complete list of changes see:
https://discourse.llvm.org/t/llvm-15-0-1-released/65380