Better tie together ContentType and Permissions. 

Since most content types (e.g. typeCode) have a 
preferred Permission, add a function to do that mapping.
Then hook up to YAML Reader and Writer such that
the 'permissions:' key does not need to be specified unless
overriding what the content type prefers.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@171929 91177308-0d34-0410-b5e6-96231b3b80d8