FileSpec: make matching separator-agnostic again

Summary:
In D18689, I removed the call to Normalize() in FileSpec::SetFile, because it no longer seemed
needed, and it resolved a quirk in the FileSpec API (spec.GetCString() returnes a path with
backslashes, but spec.GetDirectory().GetCString() has forward slashes). This turned out to be a
problem because we would consider paths with different separators as different (which led to
unresolved breakpoints for instance).

Here, I am putting back in the call to Normalize() and adding a unittest for FileSpec::Equal. I
am commenting out the GetDirectory unittests until we figure out the what is the expected
behaviour here.

Reviewers: zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D19060

llvm-svn: 266286