[OpenCL] Fix optional image types.

This change allows the use of identifiers for image types
from `cl_khr_gl_msaa_sharing` freely in the kernel code if
the extension is not supported since they are not in the
list of the reserved identifiers.

This change also removed the need for pragma for the types
in the extensions since the spec does not require the pragma
uses.

Differential Revision: https://reviews.llvm.org/D100983