Fixing a FIXME related to Unicode support on Windows. Converted the Win32 APIs...

Fixing a FIXME related to Unicode support on Windows. Converted the Win32 APIs to explicitly use the W version when it involves strings that can hold non-ASCII characters (like file paths). Now explicitly using the A version for strings that will always be ASCII (like registry key paths).

No extra tests required as this is currently covered by existing testing, and this is basically impossible to write Unicode-specific tests for.

llvm-svn: 273563