c++: __has_builtin gives the wrong answer [PR106759]

c++: __has_builtin gives the wrong answer [PR106759]

We've supported __is_nothrow_constructible since r11-4386, but
names_builtin_p didn't know about it, so it gave the wrong answer for
 #if __has_builtin(__is_nothrow_constructible)
 ...
 #endif

I've tested all C++-only built-ins and only two were missing.

	PR c++/106759

gcc/cp/ChangeLog:

	* cp-objcp-common.cc (names_builtin_p): Handle RID_IS_NOTHROW_ASSIGNABLE
	and RID_IS_NOTHROW_CONSTRUCTIBLE.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/has-builtin-1.C: New test.