Refactor implementation of 'exclude header'.

This was not a real header role, and was never exposed to clients of ModuleMap.
Remove the enumeration value for it and track it as marking the header as
'known' rather than creating an extra KnownHeader entry that *every single*
client ignores.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220460 91177308-0d34-0410-b5e6-96231b3b80d8