FIR/FIR IDE: Use entire FirVariableAssignment when reporting UNSAFE_CALL

(e.g., `nullable.a = b`), and use positioning strategies to locate the
dot in the LHS expression.

Without it, only the callee reference is reported on, which makes the
highlighting of the error and application of quickfixes incorrect in the
IDE.

Also fixed issue with annotated and/or labeled expressions on LHS of
assignment (e.g., `(@Ann label@ i) = 34`).

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/QualifiedExpressionNullability.kt

@@ -1,6 +1,7 @@
 // !DIAGNOSTICS: -DEBUG_INFO_SMARTCAST
 class Foo {
     fun foo(a: Foo): Foo = a
+    var f: Foo? = null
 }
 
 fun main() {
@@ -27,4 +28,14 @@ fun main() {
 
     val z: Foo? = null
     z!!.foo(z<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>)
+
+    val w: Foo? = null
+    w<!UNSAFE_CALL!>.<!>f = z
+    (w<!UNSAFE_CALL!>.<!>f) = z
+    (label@ w<!UNSAFE_CALL!>.<!>f) = z
+    w!!.f = z
+    w.f = z
+    w<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.f = z
+    <!SMARTCAST_IMPOSSIBLE!>w.f<!>.f = z
+    w.f!!.f = z
 }