c++: Diagnose cv-qualified decltype(auto) [PR79815]

c++: Diagnose cv-qualified decltype(auto) [PR79815]

"If the placeholder is the decltype(auto) type-specifier, T shall be the
placeholder alone." but we weren't detecting "const decltype(auto)".

I've just expanded the existing diagnostic detecting "decltype(auto) &"
and similar.

gcc/cp/ChangeLog:

	PR c++/79815
	* decl.c (grokdeclarator): Detect cv-qual decltype(auto).
	* pt.c (do_auto_deduction): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/79815
	* g++.dg/cpp1y/auto-fn59.C: New test.