Clean up llvm-config options (#3057)
Clean up llvm-config options (#3057)

Does a few small things:

1- orders our search in terms of preference where we prefer newer LLVMs
to
older ones. Generic llvm-config is considered the least desirable.

2- only search in the users PATH. don't go poking into "magic"
directories
where some tool might have installed a version looking for llvm-config.
If we do that, then it makes it impossible for a user with more than 1
LLVM installation to pick the version to use based solely on their PATH.
CausalityLtd/ponyc,dipinhora/ponyc,ponylang/ponyc,cquinn/ponyc,dipinhora/ponyc,sgebbie/ponyc,kulibali/ponyc,dipinhora/ponyc,kulibali/ponyc,kulibali/ponyc,cquinn/ponyc,jemc/ponyc,CausalityLtd/ponyc,cquinn/ponyc,sgebbie/ponyc,jemc/ponyc,cquinn/ponyc,sgebbie/ponyc,ponylang/ponyc,jemc/ponyc,sgebbie/ponyc,kulibali/ponyc,ponylang/ponyc,CausalityLtd/ponyc,sgebbie/ponyc