Split 'qualifier on reference type has no effect' out into a new flag

This introduces a new flag ignored-reference-qualifiers for the
existing "'A' qualifier on reference type B has no effect" diagnostic,
as a child of ignored-qualifiers.

Rationale:
This particular diagnostic is enabled by default, but other parts of
ignored-qualifiers are not. Anecdotally, a user may encounter this
diagnostic in the wild, and, seeing it to be valuable, might try to
raise it to error with -Werror=ignored-qualifiers, whereupon the other
diagnostics the flag covers will also be raised, to the user's surprise
and confusion. By splitting this diagnostic out into a separate flag,
and marking it as a child of ignored-qualifiers, we allow the user more
granular control of the diagnostics they care about, while maintaining
backwards compatibility with existing build scripts.