[CommandLine] Use Process::GetEnv instead of _wgetenv

Process::GetEnv does the right thing across our platforms.
CommandLine.cpp had, more or less, the same logic.  Let's remove the
duplication.

No functional change is intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@276572 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed