Bump llvm to revert accidental unbump when merging the xcode9 branch. (…

…xamarin#2828)

In 1690ccb (when merging the Xcode 9 branch
into master), llvm was accidentally debumped from
mono/llvm@6aa74ae to
mono/llvm@975e3a6.

So rebump (and one commit further, to match what we're using in the d15-4
branch).