Fix list of available OpenCL C standards

Summary:
OpenCL C is based on the C99 standard, but doesn't conform with it.
Instead there are dedicated OpenCL standards.

This is still not entirely correct, since OpenCL requires the standard
to be passed via -cl-std=... instead of -std=..., but fixing this
requires additional changes.

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5978