Add [extern_c] attribute for modules, allowing a C module to be imported within an extern "C" block in C++ code.

llvm-svn: 202615
diff --git a/clang/test/Modules/extern_c_bad.cpp b/clang/test/Modules/extern_c_bad.cpp
new file mode 100644
index 0000000..bafdc04
--- /dev/null
+++ b/clang/test/Modules/extern_c_bad.cpp
@@ -0,0 +1,2 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -verify -fmodules -x c++ -emit-module -fmodules-cache-path=%t -fmodule-name=c_library_bad %S/Inputs/module.map