Bug 272285 only affects LLVM 16, so we can restrict the dependency on devel/llvm15 to CURRENT. Additionally, the build fails on some qcoro issue on 12.4 when LLVM 15 is used with base libc++ (from LLVM 13).
While here, fix the situation with USES=compiler and USES=llvm: commit 420005fac48c3dddc7807674a5276a75d4bead77 added USES=llvm which caused issues with the existing USES=compiler:c++17-lang as reported in https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=272285#c7
The trick is to not use ${LLVM_PREFIX} when setting ${CC} and friends -- it does not seem to be set at the time Mk/Uses/compiler.mk -- we can just use relative paths.
Additionally, switch to USES=compiler:c++20-lang, as this is what the port requires in its build system.
Depends on D41045.