[OpenCL] Make Clang recognize -cl-std=1.0 as a value argument

This patch makes Clang recognize -cl-std=1.0 as a value argument,
before only -std=cl1.0 has to be used instead.

Fixes https://bugs.llvm.org/show_bug.cgi?id=47981

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D91237