...and replace it with m_last_file_spec instead.
When Source Cache is enabled, the value stored in m_last_file_sp is
already in the Source Cache, and caching it again in SourceManager
brings no extra benefit. All we need is to "remember" the last used
file, and FileSpec can serve the same purpose.
When Source Cache is disabled, the user explicitly requested no caching
of source files, and therefore, m_last_file_sp should NOT be used.