[clang][OpenCL] Fix covered switch warning

-Werror clang build is broken now.

tools/clang/lib/Sema/OpenCLBuiltins.inc:11824:5: error: default label in
switch which covers all enumeration values
[-Werror,-Wcovered-switch-default]
    default:

We don't need default now, since all enumeration values are covered.

Reviewed By: svenvh

Differential Revision: https://reviews.llvm.org/D72707