[JS IR] Generate correct source locations in sourcemap for inline declarations

* Keep returnable blocks.
* Add a new lowering which simplifies returnable blocks by introducing
  temporary variable for result and changing returnable block's type to Unit.
* Use information from returnable blocks in codegen to generate
  the right source locations in sourcemap.
* Support in namer (LocalNameGenerator).
* Fix some lowerings to work correctly with returnable blocks.

#KT-46551 In Progress

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/ReturnableBlockLowering.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -19,6 +19,7 @@ import org.jetbrains.kotlin.ir.symbols.IrReturnableBlockSymbol
 import org.jetbrains.kotlin.ir.symbols.IrSymbol
 import org.jetbrains.kotlin.ir.transformStatement
 
+// TODO migrate other usages and move this file to backend.jvm
 /**
  * Replaces returnable blocks and `return`'s with loops and `break`'s correspondingly.
  *
@@ -149,4 +150,4 @@ class ReturnableBlockTransformer(val context: CommonBackendContext, val containe
             }
         }
     }
-}
+}