Remove elements from Sema.UndefinedInternals as functions are defined. Also
filter the elements before emitting them into a PCH. No user-visible
functionality change, except that PCH files may be smaller?


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174034 91177308-0d34-0410-b5e6-96231b3b80d8