[clang] [DirectoryWatcher] Remove leading \\?\ from GetFinalPathNameByHandleW

This is needed for the paths to work when using forward slashes;
this fixes the DirectoryWatcherTests unit tests.

Also allocate missing space for the null terminator, which seems to
have been missing all along (writing the terminator out of bounds).

Differential Revision: https://reviews.llvm.org/D113264