[libclang] [OpenCL] Expose half type.

Expose the half type (fp16) through libclang and the python bindings.

It seems CXType_LastBuiltin was not updated in b2ea6d9 ("Enable
support for __float128 in Clang", 2016-04-13), so update it now.

Add an Index test for OpenCL types; in the future we will add other
OpenCL types such as images to this test.

Patch by Sven van Haastregt.

Differential Revision: https://reviews.llvm.org/D29718

llvm-svn: 294754