[FileSystem] Fix regression in FileSystem::Resolve

When I moved the resolve code from FileSpec to the FileSystem class, I
introduced a regression. If you compare the two implementations, you'll
notice that if the path doesn't exist, we should only reverse the
effects of makeAbsolute, not the effects of tilde expansion.

As a result, the logic to create the ~/.lldb directory broke, because we
would resolve the path before creating it. Because the directory didn't
exist yet, we'd call create_directories on the unresolved path, which
failed.

Differential revision: https://reviews.llvm.org/D62219

llvm-svn: 361321