In JSON output, emit a directive after metadata is generated.

To implement pipelining, Cargo needs to know when metadata generation is
finished. This commit adds code to do that. Unfortunately, metadata file
writing currently occurs very late during compilation, so pipelining
won't produce a speed-up. Moving metadata file writing earlier will be a
follow-up.

The change involves splitting the existing `Emitter::emit` method in
two: `Emitter::emit_diagnostic` and `Emitter::emit_directive`.

The JSON directives look like this:
```
{"directive":"metadata file written: liba.rmeta"}
```
The functionality is behind the `-Z emit-directives` option, and also
requires `--error-format=json`.

src/librustc_codegen_llvm/lib.rs

@@ -300,7 +300,7 @@ impl CodegenBackend for LlvmCodegenBackend {
         sess: &Session,
         dep_graph: &DepGraph,
         outputs: &OutputFilenames,
-    ) -> Result<(), ErrorReported>{
+    ) -> Result<(), ErrorReported> {
         use rustc::util::common::time;
         let (codegen_results, work_products) =
             ongoing_codegen.downcast::