[clang-cl] Check that we are in clang cl mode before enabling support for the...

[clang-cl] Check that we are in clang cl mode before enabling support for the CL environment variable.

Checking for the type of the command line tokenizer should not be the criteria to enable support for the CL environment variable, this change checks that we are in clang-cl mode instead.

Differential Revision: https://reviews.llvm.org/D23503

llvm-svn: 280702