[OpenCL] Allow -std={cl|CL}{|1.1|1.2|2.0} in driver

Fix a regression which forbids using -std=cl|CL1.1|CL1.2|CL2.0 in driver.

Allow -std and -cl-std={cl|CL}{|1.1|1.2|2.0}.

Differential Revision: http://reviews.llvm.org/D20630

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273015 91177308-0d34-0410-b5e6-96231b3b80d8