[FIR] Check top-level property initialization via CFG

Now that files have a CFG, use it to validate properties are initialized
correctly. Update FirTopLevelPropertiesChecker to collect initialization
info for the property being checked - similar to
FirMemberPropertiesChecker - and validate proper initialization.

#KT-56683 Fixed
#KT-58531 Fixed

compiler/testData/diagnostics/tests/backingField/TopLevelPropertyInitialization.fir.kt

@@ -0,0 +1,31 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_VARIABLE -ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE
+// WITH_STDLIB
+
+val a: Int
+val b = 1.also { a = 2 }
+<!MUST_BE_INITIALIZED!>val c: Int<!>
+val d by lazy { <!CAPTURED_VAL_INITIALIZATION!>c<!> = 2; 1 }
+val e: Int
+    get() {
+        <!VAL_REASSIGNMENT!>c<!> = 3
+        return c
+    }
+
+class Class {
+    val i: Int
+    val j = 1.also { i = 2 }
+    <!MUST_BE_INITIALIZED_OR_BE_ABSTRACT!>val k: Int<!>
+    val l by lazy { <!CAPTURED_MEMBER_VAL_INITIALIZATION!>k<!> = 2; 1 }
+    val n: Int
+        get() {
+            <!VAL_REASSIGNMENT!>k<!> = 3
+            return k
+        }
+}
+
+fun main() {
+    val x: Int
+    val y = 1.also { x = 2 }
+    val z: Int
+    val w by lazy { <!CAPTURED_VAL_INITIALIZATION!>z<!> = 2; 1 }
+}