Use TypeSubstitutor to get the substituted underlying type for inline

classes, instead of MemberScope.

The primary motivation was to fix issues around type-mapping for inline
classes in FIR, which uses wrapped descriptors that have empty
MemberScopes.

compiler/testData/codegen/box/inlineClasses/kt27096_nullableReference.kt

@@ -1,5 +1,4 @@
 // !LANGUAGE: +InlineClasses
-// IGNORE_BACKEND_FIR: JVM_IR
 
 inline class Z1(val x: String?)
 inline class Z2(val z: Z1)