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

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/include/ test.cl -cl-std=clc++ -emit-llvm -fdeclare-opencl-builtins -finclude-default-header -I /include/ -emit-llvm -o - `

Start by reproducing the issue with the example clang -cc1 command and inspect the libclcxx and Clang include locations used by the build. Check whether the build target or README identifies the extra headers; done means opencl_type_traits parses without missing-header errors and the required include locations are clear.

Written by the indexing model from the issue text.