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/Inputs/cxx-header.h b/clang/test/Modules/Inputs/cxx-header.h
new file mode 100644
index 0000000..7ed7775
--- /dev/null
+++ b/clang/test/Modules/Inputs/cxx-header.h
@@ -0,0 +1 @@
+int f();