Improve long path name support on Windows.

Windows normally limits the length of an absolute path name to 260
characters; directories can have lower limits.  These limits increase
to about 32K if you use absolute paths with the special '\\?\'
prefix. Teach Support\Windows\Path.inc to use that prefix as needed.

TODO: Other parts of Support could also learn to use this prefix.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221841 91177308-0d34-0410-b5e6-96231b3b80d8