[clang] Fix search path logic for C_INCLUDE_DIRS

[clang] Fix search path logic for C_INCLUDE_DIRS

For each absolute path given to C_INCLUDE_DIRS, we want it to be added
as-is to the include search path. Relative paths should be prefixed
with the sysroot.

Thanks to Marco Hinz for the patch.

Differential Revision: https://reviews.llvm.org/D69221