Debug Info: Add a testcase for clang submodule imports.

rdar://problem/17951343

Author: adrian-prantl

test/DebugInfo/ImportClangSubmodule.swift

@@ -0,0 +1,13 @@
+// RUN: rm -rf %t && mkdir -p %t
+// REQUIRES: OS=macosx
+
+// RUN: %target-swift-frontend -emit-ir %s -g -o - | FileCheck %s
+
+// CHECK: !DIImportedEntity(
+// CHECK: tag: DW_TAG_imported_module{{.*}}entity: ![[C:.*]], line: [[@LINE+1]])
+import Darwin.C
+
+let irrational = sqrt(2 as Double)
+
+// CHECK: ![[C]] = !DIModule(scope: ![[Darwin:.*]], name: "C",
+// CHECK: ![[Darwin]] = !DIModule(scope: null, name: "Darwin",