Backport Fix #1114: Verify contents of interop annotations

Backport: Only verifies @JSExport annotations unlike original commit,
since the PrepJSInterop phase does not have the facilities to easily
add the check for @JSName in 0.5.x.

(cherry picked from commit 842b0d39c416f1a074eae004717ae00870eaca2d)

Conflicts:

	compiler/src/main/scala/scala/scalajs/compiler/JSGlobalAddons.scala
	compiler/src/main/scala/scala/scalajs/compiler/PrepJSInterop.scala
	compiler/src/test/scala/scala/scalajs/compiler/test/JSInteropTest.scala