More UTF string conversion wrappers

Added new string conversion wrappers that convert between `std::string` (of UTF-8 bytes) and `std::wstring`, which is particularly useful for Win32 interop. Also fixed a missing string conversion for `getenv` on Win32, using these new wrappers.
The motivation behind this is to provide the support functions required for LLDB to work properly on Windows with non-ASCII data; however, the functions are not LLDB specific.

Patch by cameron314

Differential Revision: http://reviews.llvm.org/D17549

llvm-svn: 263247