[CommandLine] Use Process::GetEnv instead of _wgetenv
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@276572 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm