qdoc: This ends use of qdoc's old C++ parser

This change replaces the last uses of qdoc's old, ad hoc
C++ parser. Clang is now used for parsing all C++ code.
\macro, \qmlxxx, and \jsxxx commands are parsed by simple
pattern matching functions using QString::split().

Change-Id: If6f95b0487d1dd3206373bc55ec8e6b8b9c55b1e
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>