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/LValueAssignment.txt

@@ -24,6 +24,13 @@ package lvalue_assignment {
         public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
     }
 
+    @kotlin.annotation.Retention(value = AnnotationRetention.SOURCE) @kotlin.annotation.Target(allowedTargets = {AnnotationTarget.EXPRESSION}) public final annotation class Ann : kotlin.Annotation {
+        public constructor Ann()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
     public open class B {
         public constructor B()
         public final var b: kotlin.Int