Suppress various new compiler warnings.

Compilers that don't understand that elog(ERROR) doesn't return
issued warnings here.  In the cases in libpq_pipeline.c, we were
not exactly helping things by failing to mark pg_fatal() as noreturn.

Per buildfarm.

src/include/access/toast_compression.h

@@ -69,6 +69,7 @@ GetCompressionMethodName(char method)
 			return "lz4";
 		default:
 			elog(ERROR, "invalid compression method %c", method);
+			return NULL;		/* keep compiler quiet */
 	}
 }