Add Self to the name scope for a class. (#3736)

Rather than just adding `Self` to the lexical scope, add it to the
class's name scope so that it is visible in later lexical scopes for the
same class -- in particular, for out-of-line definitions of members.
Also switch some tests in `check/testdata/class` over to making
idiomatic use of `Self` both inside a class and out-of-line, now that it
works more consistently.

Note that this does not permit using `Class.Self`, but only because we
don't yet support keyword names after `.` at all. If that changed, one
could use `Class.Self` to redundantly refer to `Class`. Whether we allow
that is left to a future decision.

toolchain/check/testdata/class/basic.carbon

@@ -61,6 +61,7 @@ fn Run() -> i32 {
 // CHECK:STDOUT:   %.loc14: <unbound element of class Class> = field_decl k, element0 [template]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Class
 // CHECK:STDOUT:   .F = %F
 // CHECK:STDOUT:   .G = %G
 // CHECK:STDOUT:   .k = %.loc14