[Support] Fix FileNameLength passed to SetFileInformationByHandle

The rename_internal function used for Windows has a minor bug where the
filename length is passed as a character count instead of a byte count.
Windows internally ignores this field, but other tools that hook NT
api's may use the documented behavior:

MSDN documentation specifying the size should be in bytes:
https://docs.microsoft.com/en-us/windows/desktop/api/winbase/ns-winbase-_file_rename_info

Patch by Ben Hillis.

Differential Revision: https://reviews.llvm.org/D55624

llvm-svn: 348995