Fix unit tests on Windows: handle env vars with non-ASCII chars.

Summary: On Windows we have to take UTF16 encoded env vars and convert them to UTF8. This patch fixes CopyEnvironment helper function used by process unit tests.

Reviewers: yaron.keren

Subscribers: yaron.keren, llvm-commits

Differential Revision: http://reviews.llvm.org/D14278

llvm-svn: 252039