Allow extend adapt of non-class types. (#4544)

Stop rejecting `extend adapt` of non-class types such as struct and
tuple. These don't actually work just yet because name lookup into
struct and tuple types is a special case that doesn't handle adapters,
but this gets us a bit closer.

This also slightly improves error recovery for name lookup into an
invalid scope.

toolchain/check/testdata/pointer/fail_deref_function.carbon

@@ -49,6 +49,7 @@ fn A() {
 // CHECK:STDOUT:   %.loc16: ref <error> = deref <error>
 // CHECK:STDOUT:   %A.ref.loc20: %A.type = name_ref A, file.%A.decl [template = constants.%A]
 // CHECK:STDOUT:   %.loc20: ref <error> = deref <error>
+// CHECK:STDOUT:   %foo.ref: <error> = name_ref foo, <error> [template = <error>]
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT: