Unextend Char from Number

This is impossible to achieve on JVM, where java/lang/Character does not extend
java/lang/Number

compiler/testData/diagnostics/tests/numbers/characterIsNotANumber.kt

@@ -0,0 +1,11 @@
+fun foo(n: Number) = n
+
+fun test() {
+    foo(<!CONSTANT_EXPECTED_TYPE_MISMATCH!>'a'<!>)
+    
+    val c = 'c'
+    foo(<!TYPE_MISMATCH!>c<!>)
+
+    val d: Char? = 'd'
+    foo(<!TYPE_MISMATCH!>d<!>!!)
+}