clang -cc1 fails to parse opencl_type_traits due to missing header files

[AnastasiaStulova]

When clang is used with -cc1 parsing opencl_type_traits ends up with missing include files, for example features.h.

Do we need to update build target or readme to specify the location of extra headers?

Example command line to reproduce failures:

clang -cc1 -I<libclcxx build location>/include/ test.cl -cl-std=clc++ -emit-llvm -fdeclare-opencl-builtins -finclude-default-header -I <clang build location>/include/ -emit-llvm -o -