FileSpec objects that resolve to "." should have "." in m_filename and m_directory empty.

After switching to LLVM normalization, if we init FileSpec with "." we would end up with m_directory being NULL and m_filename being "".

This patch fixes this by allowing the path to be normalized and if it normalized to nothing, set it to m_filename.

Differential Revision: https://reviews.llvm.org/D46783

llvm-svn: 332618