c++: Detect long double -> double narrowing [PR94590]

c++: Detect long double -> double narrowing [PR94590]

This PR points out that we don't detect long double -> double narrowing
when long double happens to have the same precision as double; on x86_64
this can be achieved by -mlong-double-64.

[dcl.init.list]#7.2 specifically says "from long double to double or float,
or from double to float", but check_narrowing only checks

  TYPE_PRECISION (type) < TYPE_PRECISION (ftype)

so we need to handle the other cases too, e.g. by same_type_p as in
the following patch.

	PR c++/94590 - Detect long double -> double narrowing.
	* typeck2.c (check_narrowing): Detect long double -> double
	narrowing even when double and long double have the same
	precision.  Make it handle conversions to float too.

	* g++.dg/cpp0x/Wnarrowing18.C: New test.