Add test case for imported conformance usage after Sema tear-down

This test used to fail because we would look up an associated type
in an incomplete conformance after Sema was torn down, causing a
crash.

Now, the ClangImporter is able to correctly complete the conformance.

Author: slavapestov

test/multifile/imported-conformance/option-set/library.swift

@@ -0,0 +1,8 @@
+// RUN: true
+
+import Foundation
+
+@inline(__always)
+public func replace(rgx: NSRegularExpression, in: String, with: String, x: NSRange) -> String {
+  return rgx.stringByReplacingMatches(in: `in`, options: [], range: x, withTemplate: with)
+}

test/multifile/imported-conformance/option-set/main.swift

@@ -0,0 +1,14 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %empty-directory(%t/linker)
+// RUN: %target-build-swift -emit-module -emit-library %S/library.swift -o %t/linker/liblibrary.%target-dylib-extension -emit-module-path %t/linker/library.swiftmodule -module-name library
+// RUN: %target-build-swift %S/main.swift -I %t/linker/ -L %t/linker/ -llibrary -o %t/linker/main
+
+// REQUIRES: executable_test
+
+import Foundation
+import library
+
+public func rreplace(rgx: NSRegularExpression, in: String, with: String, x: NSRange) -> String {
+  return replace(rgx: rgx, in: `in`, with: with, x: x)
+}