Add a test case for SR-11227 (extending a protocol composition).

Author: varungandhi-apple

test/Sema/Inputs/composition_extensions_usage.swift

@@ -0,0 +1,10 @@
+struct S {}
+
+extension S : P2 {}
+
+extension S : P1_1 {}
+
+func f() {
+  let s = S.init()
+  s.p1_1()
+}

test/Sema/composition_extensions.swift

@@ -0,0 +1,19 @@
+// XFAIL: *
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -primary-file %s %S/Inputs/composition_extensions_usage.swift -emit-module-path %t/P-partial.swiftmodule -module-name SR11227 -enable-testing
+// RUN: %target-swift-frontend -primary-file %S/Inputs/composition_extensions_usage.swift %s -emit-module-path %t/S-partial.swiftmodule -module-name SR11227 -enable-testing
+// RUN: %target-swift-frontend -sil-merge-partial-modules %t/P-partial.swiftmodule %t/S-partial.swiftmodule -emit-module -o %t/SR11227.swiftmodule
+
+protocol P0 {}
+
+protocol P1 {}
+
+protocol P1_1 : P1 {
+  func p1_1()
+}
+
+protocol P2 {}
+
+extension P1 & P1_1 where Self : P2 {
+  func p1_1() {}
+}