CC Mode: Prevent rapid alternation of fontification of "found types"

CC Mode: Prevent rapid alternation of fontification of "found types"

This fixes bug #52863.

* lisp/progmodes/cc-engine.el (c-forward-decl-or-cast-1): When a new type is
found, postpone entering it into c-found-types (and thus triggering the
fontification of that type throughout the buffer) until the end of the
function, when we're sure that the "type" found actually is a type.