c++: Don't quote nothrow in diagnostic

c++: Don't quote nothrow in diagnostic

In <https://gcc.gnu.org/pipermail/gcc-patches/2022-September/602057.html>
Jason noticed that we quote "nothrow" in diagnostics even though it's
not a keyword in C++.  This patch removes the quotes and also drops
"nothrow" from c_keywords.

gcc/c-family/ChangeLog:

	* c-format.cc (c_keywords): Drop nothrow.

gcc/cp/ChangeLog:

	* constraint.cc (diagnose_trait_expr): Say "nothrow" without quotes
	rather than in quotes.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-traits3.C: Adjust expected diagnostics.