RequirementMachine: Add redundant requirements diagnostic test that used to fail with the GSB

For some reason we didn't diagnose some of the redundancies here;
the new logic in the Requirement Machine works just fine though!

https://bugs.swift.org/browse/SR-14917 / rdar://problem/80820294

Author: slavapestov

test/Generics/sr14917.swift

@@ -0,0 +1,27 @@
+// RUN: %target-typecheck-verify-swift -requirement-machine-inferred-signatures=on
+
+protocol P {
+  associatedtype A
+  associatedtype AS: Q where AS.B == A
+}
+
+protocol Q {
+  associatedtype B
+}
+
+struct S1<T : P> where T.AS.B == T.A {}
+// expected-warning@-1 {{redundant same-type constraint 'T.AS.B' == 'T.A'}}
+
+struct S2<T: P> {
+  struct Nested where T.AS.B == T.A {}
+  // expected-warning@-1 {{redundant same-type constraint 'T.AS.B' == 'T.A'}}
+}
+
+extension S2 where T.AS.B == T.A {}
+// expected-warning@-1 {{redundant same-type constraint 'T.AS.B' == 'T.A'}}
+
+extension P where AS.B == A {}
+// expected-warning@-1 {{redundant same-type constraint 'Self.AS.B' == 'Self.A'}}
+
+extension P where Self : P {}
+// expected-warning@-1 {{redundant conformance constraint 'Self' : 'P'}}