[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