c++: Implement -Wctad-maybe-unsupported.

c++: Implement -Wctad-maybe-unsupported.

I noticed that clang++ has this CTAD warning and thought that it might
be useful to have it.  From clang++: "Some style guides want to allow
using CTAD only on types that "opt-in"; i.e. on types that are designed
to support it and not just types that *happen* to work with it."

So this warning warns when CTAD deduced a type, but the type does not
define any deduction guides.  In that case CTAD worked only because the
compiler synthesized the implicit deduction guides.  That might not be
intended.

It can be suppressed by adding a deduction guide that will never be
considered:

  struct allow_ctad_t;
  template <typename T> struct S { S(T) {} };
  S(allow_ctad_t) -> S<void>;

This warning is off by default.  It doesn't warn when the type comes
from a system header unless -Wsystem-headers.

gcc/c-family/ChangeLog:

	* c.opt (Wctad-maybe-unsupported): New option.

gcc/cp/ChangeLog:

	* pt.c (deduction_guides_for): Add a bool parameter.  Set it.
	(do_class_deduction): Warn when CTAD succeeds but the type doesn't
	have any explicit deduction guides.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wctad-maybe-unsupported.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wctad-maybe-unsupported.C: New test.
	* g++.dg/warn/Wctad-maybe-unsupported2.C: New test.
	* g++.dg/warn/Wctad-maybe-unsupported3.C: New test.
	* g++.dg/warn/Wctad-maybe-unsupported.h: New file.