IR: make call symbols mutable

This will help in some lowerings which need to change the callee of a
call. Instead of creating a new call and copying everything (type
arguments, value arguments, receivers, annotations, attributes, ...),
it's easier to modify the `symbol`. In a way, this is a continuation of
22b4b29292.

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

@@ -881,7 +881,7 @@ class FunctionInlining(
     }
 
     private class IrGetValueWithoutLocation(
-        override val symbol: IrValueSymbol,
+        override var symbol: IrValueSymbol,
         override var origin: IrStatementOrigin? = null
     ) : IrGetValue() {
         override val startOffset: Int get() = UNDEFINED_OFFSET