Refactor and simplify DeclUpdates serialization.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204397 91177308-0d34-0410-b5e6-96231b3b80d8
When the exception specification for a function in an imported PCH or module is
resolved, emit an update record.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204403 91177308-0d34-0410-b5e6-96231b3b80d8