JVM_IR: KT-40330 Unify field names for captured 'this' with JVM

NB some cases such as captured extension receiver for an extension
lambda are not supported yet; to be discussed, to what extent should we
actually follow JVM code shape here.

compiler/testData/codegen/bytecodeText/mangling/parentheses.kt

@@ -17,9 +17,10 @@ fun box(): String {
     return `(X)`().`(Y)`()
 }
 
+// JVM_TEMPLATES
 // One instance of each is in kotlin.Metadata.d2
 // 1 \(X\)
 // 1 \(Y\)
 
 // JVM_IR_TEMPLATES
-// 4 \$this
+// 4 this\$0