FIR: Do not create incorrect synthetic property

compiler/testData/diagnostics/tests/syntheticExtensions/javaProperties/nonValidFirstChar.kt

@@ -0,0 +1,14 @@
+// FIR_IDENTICAL
+// SKIP_TXT
+// FILE: A.java
+public class A {
+    public String get1() {
+        return "";
+    }
+}
+
+// FILE: main.kt
+fun foo(a: A) {
+    a.<!UNRESOLVED_REFERENCE!>`1`<!>
+    a.get1()
+}