C++ Mode: handle __attribute__,etc. inside constructor argument lists

C++ Mode: handle __attribute__,etc. inside constructor argument lists

This corrects both the fontification and indentation of these things, fixing
bug #42270.

* lisp/progmodes/cc-engine.el (c-do-declarators): Skip over "hangon keys" and
noise macros whilst scanning a putative C++ function.
(c-forward-decl-or-cast-1): When checking for typeless functions, skip over
"hangon keys" and noise macros.

* lisp/progmodes/cc-mode.el (c-fl-decl-end): Deal with certain invalid
"nested declarators" by scanning over them with a recursive call of
c-fl-decl-end.

* lisp/progmodes/cc-vars.el (c-noise-macro-names)
(c-noise-macro-with-parens-names): State in the doc strings that if either of
these is a regexp, it must have a submatch 1 which matches the noise macro
exactly.