Add a multifile test that a dependency on an external protocol

conformance will block compilation if an associated type is invalid.

Author: rjmccall

test/multifile/Inputs/external-protocol-conformance/A.swift

@@ -0,0 +1,10 @@
+protocol PHelper {
+}
+
+protocol P {
+  associatedtype Assoc: PHelper // expected-note {{protocol requires nested type 'Assoc'}}
+}
+
+struct A : P { // expected-error {{type 'A' does not conform to protocol 'P'}}
+  typealias Assoc = Int // expected-note {{possibly intended match 'Assoc' (aka 'Int') does not conform to 'PHelper'}}
+}

test/multifile/external-protocol-conformance.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-frontend -emit-ir -verify %s %S/Inputs/external-protocol-conformance/A.swift
+
+func generic<T: P>(value: T) {}
+
+// The goal here is to verify that a dependency on an invalid associated
+// type in a protocol conformance in another file will prevent code
+// generation.
+func useGeneric() {
+  generic(value: A())
+}