[lldb] Fix SBFileSpec.fullpath

Reimplement `SBFileSpec.fullpath` to (indirectly) use `FileSpec::GetPath`.

Instead of hardcoding a `/` separator, use `GetPath`. This makes use of the
`FileSpec`'s internal style, which for example allows for backslash on Windows
where required.

It's not obvious from looking at the source, but the `fullpath` property is
implemented with `str`, which calls `GetDescription`, which finally calls
`GetPath`.

Differential Revision: https://reviews.llvm.org/D138348