self-profiling: Record something more useful for crate metadata generation event.

Before this commit, we had an event that would only track the compression step
for proc-macros and Rust dylibs. After the commit we measure the time for
acutally generating the crate metadata bytes.

src/librustc/ty/context.rs

@@ -1408,6 +1408,7 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     pub fn encode_metadata(self)-> EncodedMetadata {
+        let _prof_timer = self.prof.generic_activity("generate_crate_metadata");
         self.cstore.encode_metadata(self)
     }
 

src/librustc_codegen_ssa/base.rs

@@ -574,8 +574,6 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
 
     if need_metadata_module {
         // Codegen the encoded metadata.
-        let _prof_timer = tcx.prof.generic_activity("codegen_crate_metadata");
-
         let metadata_cgu_name = cgu_name_builder.build_cgu_name(LOCAL_CRATE,
                                                                 &["crate"],
                                                                 Some("metadata")).as_str()