[Support] Improve Windows widenPath and add support for long UNC paths

Check the path length limit against the length of the UTF-16 version of
the input rather than the UTF-8 equivalent, as the UTF-16 length may be
shorter. Move widenPath from the llvm::sys::path namespace in Path.h to
the llvm::sys::windows namespace in WindowsSupport.h. Only use the
reduced path length limit for create directory. Canonicalize using
sys::path::remove_dots().

Differential Revision: https://reviews.llvm.org/D75372