libstdc++: Fix flat_map::operator[] for const lvalue keys [PR120432]

libstdc++: Fix flat_map::operator[] for const lvalue keys [PR120432]

The const lvalue operator[] overload wasn't properly forwarding the key
type to the generic overload, causing a hard error for const keys.

Rather than correcting the forwarded type this patch just makes the
non-template overloads call try_emplace directly instead.  That way we
can remove the non-standard same_as constraint on the generic overload
and match the spec more closely.

	PR libstdc++/120432

libstdc++-v3/ChangeLog:

	* include/std/flat_map (flat_map::operator[]): Make the
	non-template overloads call try_emplace directly.  Remove
	non-standard same_as constraint on the template overload.
	* testsuite/23_containers/flat_map/1.cc (test08): New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
(cherry picked from commit 91ed3248ce26aaaee4d7471aa4edbc07b3f1a90e)