[FIR] take ready implicit unit type instead of return type calculation

and assert that symbol is not a substitution/intersection override
in the `compute` method otherwise.

Because `fakeOverrideSubstitution` should be calculated for all real
implicit types, no call to this method should actually happen.

Otherwise, it can be problematic to create a session
which would contain the full designation path:
`provider.getFirCallableContainerFile(symbol)`
returns `firFile` of a super class which might be from module `a`,
when declaration and its outer classes are from module `b`.

^KTIJ-24105

analysis/analysis-api/testData/components/callResolver/resolveCall/substituteOverrideResolution.kt

@@ -0,0 +1,12 @@
+// WITH_STDLIB
+package main
+
+import kotlin.collections.MutableList
+
+abstract class B : MutableList<String>() {}
+
+fun usage(b : B?) {
+    if (b != null) {
+        <expr>b.clear()</expr>
+    }
+}