Revert "Frontend: Extract SerializedDiagnosticReader out of CXLoadedDiagnostic (NFC)"

The bots can't seem to find an include file. Reverting for now and
I'll look into it in a bit.

This reverts commits r219647 and r219648.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219649 91177308-0d34-0410-b5e6-96231b3b80d8