Generate IR protobuf classes as multiple Java files

Otherwise KotlinIr.java becomes too big for IDEA to swallow in the
default configuration.

compiler/ir/serialization.common/src/KotlinIr.proto

@@ -1,6 +1,7 @@
 syntax = "proto2";
-package org.jetbrains.kotlin.backend.common.serialization;
+package org.jetbrains.kotlin.backend.common.serialization.proto;
 
+option java_multiple_files = true;
 option java_outer_classname = "KotlinIr";
 option optimize_for = LITE_RUNTIME;