Improve ".." handling in FileSpec normalization

Summary:
.. handling for windows path was completely broken because the function was
expecting \ as path separators, but we were passing it normalized file paths,
where these have been replaced by forward slashes. Apart from this, the function
was incorrect for posix paths as well in some corner cases, as well as being
generally hard to follow.

The corner cases were:
- /../bar -> should be same as /bar
- /bar/.. -> should be same as / (slightly dodgy as the former depends on /bar actually
  existing, but since we're doing it in an abstract way, I think the
  transformation is reasonable)

I rewrite the function to fix these corner cases and handle windows paths more
correctly. The function should now handle the posix paths (modulo symlinks, but
we cannot really do anything about that without a real filesystem). For windows
paths, there are a couple of corner cases left, mostly to do with drive letter
handling, which cannot be fixed until the rest of the class understands drive
letters better.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D26081

llvm-svn: 285593