FIR: Rework FirRegularTowerDataContexts

- Get rid of SPECIAL mode (just left REGULAR and class-related instead)
- Clear naming
- Restore contexts after lambda/callable reference are processed

The test has been failing before this change because after
callable reference is resolved, its tower data context has been left
erroneously in the SPECIAL-related entry

compiler/testData/diagnostics/tests/dataFlow/smartCastWithLambdaAndCallableReference.fir.kt

@@ -0,0 +1,21 @@
+// SKIP_TXT
+
+fun <T> T.myApply(block: T.() -> Unit): T = this
+
+fun bar(): Int = 1
+
+interface A : C
+interface B : C
+interface C {
+    fun baz()
+}
+
+fun Any.foo() = myApply {
+    when (this) {
+        is A -> ::bar
+        is B -> ::bar
+        else -> throw RuntimeException()
+    }
+
+    baz() // Smart cast should work
+}