Fix an indexer bug involving files that are included but contain no refs.

 * There are two places in the code that operate on Path symbols:

    - When a reference is added, an IndexerFileContext is constructed for
      the file, and a Path symbol is created for the IndexerFileContext.
      When this happens, the new symbol is recorded as type "Path".  (This
      commit has a non-functional change here that uses
      getSymbolTypeID(ST_Path) instead of insertSymbolType("Path").)

    - When a file includes another file, the target path is converted to a
      symbol, but no IndexerFileContext is created for the target path.
      Previously, it was possible that the target path symbol would not be
      marked with any type, so the path symbol would not appear in the
      navigator GUI's file lists.  Fix this by calling recordSymbol in
      IndexerPPCallbacks::InclusionDirective.