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