[clang-cl] Add support for CL and _CL_ environment variables

cl uses 'CL' and '_CL_' to prepend and append command line options to
the given argument vector.  There is an additional quirk whereby '#' is
transformed into '='.

Differential Revision: http://reviews.llvm.org/D11896

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244473 91177308-0d34-0410-b5e6-96231b3b80d8