llvm: fix llvm-config
llvm: fix llvm-config

`llvm-config` relies on a versioned `libLLVM` (e.g. `libLLVM-12`), but
their build system does not install one for some reason.

With the symlink:

    ❯ llvm-config --libs
    -lLLVM-12

Without:

    ❯ llvm-config --libs
    -lLLVMWindowsManifest -lLLVMXRay -lLLVMLibDriver -lLLVMDlltoolDriver ... -lLLVMRemarks -lLLVMBitstreamReader -lLLVMBinaryFormat -lLLVMTableGen -lLLVMSupport -lLLVMDemangle

This deviates from the intended behaviour, as seen on Linux, where
`llvm-config --libs` returns the output shown above with the symlink.
Attempting to link against the libraries shown by `llvm-config` without
this fix is also leads to build failures due to missing symbols.

This will also allow Julia to use brewed `llvm` instead of a vendored
one. (#76527)

Closes #76798.

Closes #76646.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>
Signed-off-by: BrewTestBot <8a898ee6867e4f2028e63d2a6319b2224641c06c@users.noreply.github.com>

stgraber/homebrew-core,adam-moss/homebrew-core,JCount/homebrew-core,nandub/homebrew-core,zyedidia/homebrew-core,JCount/homebrew-core,lemzwerg/homebrew-core,mfikes/homebrew-core,cblecker/homebrew-core,mbcoguno/homebrew-core,zyedidia/homebrew-core,filcab/homebrew-core,adam-moss/homebrew-core,chrisfinazzo/homebrew-core,sjackman/homebrew-core,nbari/homebrew-core,uyjulian/homebrew-core,sjackman/homebrew-core,Linuxbrew/homebrew-core,makigumo/homebrew-core,Homebrew/homebrew-core,filcab/homebrew-core,mfikes/homebrew-core,makigumo/homebrew-core,Moisan/homebrew-core,Linuxbrew/homebrew-core,Homebrew/homebrew-core,mbcoguno/homebrew-core,Moisan/homebrew-core,chrisfinazzo/homebrew-core,nbari/homebrew-core,lemzwerg/homebrew-core,stgraber/homebrew-core,mbcoguno/homebrew-core,uyjulian/homebrew-core,cblecker/homebrew-core,nandub/homebrew-core