[Serialization] Don't look at extension members when resolving XREFs.

This is mostly a "don't crash" commit, but since member XREFs don't
specify which module they're looking in, they can actually pick up
members from the module currently being compiled...which may not have
a type yet.

rdar://problem/21071045

Swift SVN r30895

test/Serialization/xref-multi-file.swift

@@ -0,0 +1,15 @@
+// RUN: rm -rf %t && mkdir %t
+// RUN: %target-swift-frontend -emit-module -o %t %S/Inputs/struct_with_operators.swift
+// RUN: %target-swift-frontend -emit-module -o %t %S/Inputs/alias.swift -module-name has_alias
+// RUN: %target-swift-frontend -emit-module -o %t -I %t %S/Inputs/has_xref.swift
+// RUN: llvm-bcanalyzer %t/has_xref.swiftmodule | FileCheck %s
+// RUN: %target-swift-frontend -emit-silgen -I %t -primary-file %s %S/Inputs/xref-multi-file-other.swift -module-name main > /dev/null
+
+// CHECK-NOT: UnknownCode
+
+import has_xref
+
+func use<T: DefaultInitializable>(_: T) {}
+func test(x: SpecialInt) {
+  use(x)
+}