Because processing these commands do not use clang, their implementation
is moved from ClangCodeParser to CppCodeParser - this ensures
that these topic commands are processed also when they appear in
.qdoc files (PureDocParser, used for .qdoc, derives from CppCodeParser).

Also, these functions did nothing when qdoc was running in prepare
phase which in turn meant that they were not written to .index, thus
breaking cross-module linking for macros/QML methods/signals. That
check is now removed and the functions always do their thing.

Change-Id: I8d9333b02092708955b619b573254eda5d82f038
Reviewed-by: Martin Smith <martin.smith@qt.io>