[Support][Windows] Fix incorrect GetFinalPathNameByHandleW() return value...

[Support][Windows] Fix incorrect GetFinalPathNameByHandleW() return value check in realPathFromHandle()

`GetFinalPathNameByHandleW(,,N,)` returns:
- `< N` on success (this value does not include the size of the terminating null character)
- `>= N` if buffer is too small (this value includes the size of the terminating null character)

So, when `N == Buffer.capacity() - 1`, we need to resize buffer if return value is > `Buffer.capacity() - 2`.
Also, we can set `N` to `Buffer.capacity()`.

Thus, without this patch `realPathFromHandle()` returns unfilled buffer when length of the final path of the file is equal to `Buffer.capacity()` or `Buffer.capacity() - 1`.

Reviewed By: andrewng, amccarth

Differential Revision: https://reviews.llvm.org/D86564