[cxx-interop][nfc] Add a test case for linker error.

Author: zoecarver

test/Interop/Cxx/extern-c/Inputs/inline-func.h

@@ -0,0 +1,18 @@
+extern "C" {
+
+inline void inlineFn();
+
+void cacheMis() { }
+void incorrectCacheHit() {
+  inlineFn();
+}
+
+static void caller() {
+  cacheMis();
+  incorrectCacheHit();
+}
+
+inline void inlineFn() { }
+
+}
+

test/Interop/Cxx/extern-c/Inputs/module.modulemap

@@ -2,3 +2,8 @@ module ExternC {
   header "extern-c.h"
   requires cplusplus
 }
+
+module InlineFunc {
+  header "inline-func.h"
+  requires cplusplus
+}

test/Interop/Cxx/extern-c/inline-func.swift

@@ -0,0 +1,8 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs/ -Xfrontend -enable-experimental-cxx-interop)
+// REQUIRES: executable_test
+
+import InlineFunc
+
+public func test() {
+  caller()
+}