Apple also contributes plenty to LLVM, more than Intel actually, naively based on commit counts of @apple.com and @intel.com git committer email addresses. This isn't very surprising given that Chris Lattner worked at Apple for over a decade.
LLVM is Apple's baby, beyond its genesis under Lattner. They hate the GPL, that's it.
The thing about their contributions is that they upstream stuff they want other people to standardize on but aren't doing it out of love, as far as I can tell e.g. Valve have a direct business interest in having Linux kicking ass, Apple actively loses out (psychologically at least) if a non-apple toolchain is as good as theirs.
