[FIR] Don't approximate captured types

This fixes some type argument mismatch errors caused by a captured type
being approximated and then captured again.
Some places need to be adapted to work with captured types that
previously only worked with approximated types.

#KT-62959 Fixed

compiler/testData/diagnostics/tests/sealed/ExhaustiveOnCaptured.kt

@@ -0,0 +1,18 @@
+// FIR_IDENTICAL
+
+abstract class FirBasedSymbol<E : FirDeclaration> {
+    val fir: E get() = null!!
+}
+abstract class FirCallableSymbol<D : FirCallableDeclaration> : FirBasedSymbol<D>()
+
+sealed class FirDeclaration
+sealed class FirCallableDeclaration : FirDeclaration()
+
+class FirFunction : FirCallableDeclaration()
+class FirVariable : FirCallableDeclaration()
+
+val FirCallableSymbol<*>.isExtension: Boolean
+    get() = when (fir) {
+        is FirFunction -> true
+        is FirVariable -> false
+    }