NCGenerics: add more extension test coverage

Author: kavon

test/Generics/inverse_extensions.swift

@@ -0,0 +1,22 @@
+// RUN: %target-typecheck-verify-swift \
+// RUN:   -enable-experimental-feature NoncopyableGenerics \
+// RUN:   -enable-experimental-feature NonescapableTypes
+
+struct Turtle<T> {}
+extension Turtle where T: ~Copyable {} // expected-error {{'T' required to be 'Copyable' but is marked with '~Copyable'}}
+
+struct Rabbit<T> where T: ~Copyable {}
+extension Rabbit where T: ~Escapable {} // expected-error {{'T' required to be 'Escapable' but is marked with '~Escapable'}}
+
+protocol P {}
+extension P where Self: ~Escapable {} // expected-error {{'Self' required to be 'Escapable' but is marked with '~Escapable'}}
+
+protocol HasAssoc {
+  associatedtype A
+}
+extension HasAssoc where Self.A: ~Copyable {}
+// expected-error@-1 {{cannot add inverse constraint 'Self.A: ~Copyable' on generic parameter 'Self.A' defined in outer scope}}
+// expected-error@-2 {{'Self.A' required to be 'Copyable' but is marked with '~Copyable'}}
+
+class Box<T: ~Copyable> {}
+extension Box where T: ~Copyable {}