c++: Fix wrong paren-init of aggregates interference [PR93790]

c++: Fix wrong paren-init of aggregates interference [PR93790]

This PR points out that we are rejecting valid code in C++20.  The
problem is that we were surreptitiously transforming

  T& t(e)

into

  T& t{e}

which is wrong, because the type of e had a conversion function to T,
while aggregate initialization of t from e doesn't work.  Therefore, I
was violating a design principle of P0960, which says that any existing
meaning of A(b) should not change.  So I think we should only attempt to
aggregate-initialize if the original expression was ill-formed.

Another design principle is that () should work where {} works, so this:

  struct S { int i; };
  const S& s(1);

has to keep working.  Thus the special handling for paren-lists with one
element.  (A paren-list with more than one element would give you "error:
expression list treated as compound expression in initializer" C++17.)

	PR c++/93790
	* call.c (initialize_reference): If the reference binding failed, maybe
	try initializing from { }.
	* decl.c (grok_reference_init): For T& t(e), set
	LOOKUP_AGGREGATE_PAREN_INIT but don't build up a constructor yet.

	* g++.dg/cpp2a/paren-init23.C: New test.
	* g++.dg/init/aggr14.C: New test.