JVM_IR: use ConcurrentHashMap

Replace mutable maps and sets accessed from by-file lowerings with
ConcurrentHashMap, so that lowerings can operate on them in parallel.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/Mappings.kt

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.backend.common
 
 import org.jetbrains.kotlin.ir.declarations.*
+import java.util.concurrent.ConcurrentHashMap
 import kotlin.reflect.KMutableProperty0
 import kotlin.reflect.KProperty
 
@@ -47,7 +48,7 @@ open class DefaultMapping : Mapping {
     override val reflectedNameAccessor: Mapping.Delegate<IrClass, IrSimpleFunction> = newMapping()
 
     protected open fun <K : IrDeclaration, V> newMapping() = object : Mapping.Delegate<K, V>() {
-        private val map: MutableMap<K, V> = mutableMapOf()
+        private val map: MutableMap<K, V> = ConcurrentHashMap()
 
         override operator fun get(key: K): V? {
             return map[key]