[OpenCL] Reword unknown extension pragma diagnostic

For newer OpenCL extensions that do not require a pragma, such as
`cl_khr_subgroup_shuffle`, a user could still accidentally attempt to
use a pragma.  This would result in a warning
  "unknown OpenCL extension 'cl_khr_subgroup_shuffle' - ignoring"
which could be mistakenly interpreted as "clang does not support this
extension at all" instead of "clang does not require any pragma for
this extension".

Differential Revision: https://reviews.llvm.org/D126660