For some reason, sometimes the directory paths that clang emits have internal
relative paths, like:

/whatever/llvm/lib/Sema/../../include/llvm/Sema/

That causes problems with our type uniquing, since we use the declaration file
and line as one component of the uniquing, and different ways of getting to the
same file will have different directory spellings, though they are functionally
equivalent.  We end up with two copies of the exact same type because of this,
and that makes the expression parser give "duplicate type" errors.

I added a method to resolve paths with ../ in them and used that in the FileSpec::Equals,
for comparing Declarations and for doing Breakpoint compares as well, since they also
suffer from this if you specify breakpoints by full path (since nobody knows what
../'s to insert...)

<rdar://problem/18765814>