Allow overriding of Clang prebuilts

These are the environment variables used by make to override the
llvm/clang prebuilts. As far as I know, these should only be used when
building or testing a new clang drop.

I would have this handled by make and put in soong.variables, but I
expect this to be one of the first options removed from make and
exported from Soong to Kati.

Change-Id: I7a3a70ee44fc4b1e56a5e200e2a7ced77d4c5692
1 file changed