Support bound references to objects and companion objects

Tweak member extension detection a little bit to prohibit extensions imported
from objects (they don't have one of the receiver parameters)

compiler/testData/diagnostics/tests/callableReference/bound/object.kt

@@ -0,0 +1,11 @@
+// !CHECK_TYPE
+
+object Obj {
+    fun foo() {}
+    val bar = 2
+}
+
+fun test() {
+    checkSubtype<() -> Unit>(Obj::foo)
+    checkSubtype<() -> Int>(Obj::bar)
+}