[OpenCL] Prevent adding extension pragma by default.

[OpenCL] Prevent adding extension pragma by default.

This commit refactors extension support to allow
specifying whether pragma is needed or not explicitly.

For backward compatibility pragmas are set to required
for all extensions that were added prior to this but
not for OpenCL 3.0 features.

Differential Revision: https://reviews.llvm.org/D97052