Install LLVM 10 for OSX CI

[bcardiff]

LLVM 10 has been releases on homebrew.

The osx ci is failing since LLVM_CONFIG environment variable in test_darwin points to a non-aliased llvm-config path. Either that path or the formula version needs to change when a new llvm formula version is released in homebrew for now.

[Sija]

Shouldn't this be on par with the homebrew formula?

Suggested change

	on_osx brew install llvm@10 gmp libevent pcre openssl pkg-config
	on_osx brew link --force llvm@10
	on_osx brew install llvm gmp libevent pcre openssl pkg-config
	on_osx brew link --force llvm

[bcardiff]

I prefer to use the alias to a fixed version and bump it manually when we are ready. Otherwise until we have support for the next llvm version we would need to add the @V as a fix.

Ideally the LLVM_CONFIG path should work on any alias picked in this line. But since we already support llvm@10, this was the shortest change.

[yxhuvud]

One thing I note is that the definition of LLVM_CONFIG_FINDER in /Makefile hasn't been updated for a while. That seems .. related. How do these things interact?

[RX14]

Would be nice if we fixed the CI scripts to set LLVM_CONFIG correctly regardless of if it's an old formula or no.