[test] Add a test for SR-156. (#2262)

Author: jrose-apple

test/multifile/Inputs/protocol-conformance-member-helper.swift

@@ -0,0 +1,3 @@
+struct CoolStruct {
+  let coolFactor: Double
+}

test/multifile/protocol-conformance-member.swift

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+// RUN: rm -rf %t && mkdir %t
+// RUN: %target-build-swift -emit-library %s %S/Inputs/protocol-conformance-member-helper.swift -o %t/libTest.dylib -module-name Test
+// RUN: nm %t/libTest.dylib | FileCheck %s
+
+// CHECK: _TFV4Test10CoolStructg10coolFactorSd
+
+// SR-156: Make sure we synthesize getters for members used as protocol
+// witnesses. Check that we link correctly; we don't care which file
+// synthesizes it.
+
+protocol CoolStructProtocol {
+  var coolFactor: Double { get }
+}
+extension CoolStruct : CoolStructProtocol {}