Add support for CPATH and friends.

This moves the existing code for CPATH into the driver and adds the environment lookup and path splitting there.
The paths are then passed down to cc1 with -I options (CPATH), added after the normal user-specified include dirs.
Language specific paths are passed via -LANG-isystem and the actual filtering is performed in the frontend.

I tried to match GCC's behavior as close as possible

Fixes PR8971.

llvm-svn: 140341