[OpenCL] Simplify use of C11 atomic types.

Remove requirements on extension pragma in atomic types
because it has not respected the spec wrt disabling types
and hasn't been useful either. With this change, the
developers can use atomic types from the extensions if they
are supported without enabling the pragma just like the builtin
functions

This patch does not break backward compatibility since the
extension pragma is still supported and it makes the behavior of
the compiler less strict by accepting code without needless and
inconsistent pragma statements.

Differential Revision: https://reviews.llvm.org/D100976