Re-apply "Frontend: Extract SerializedDiagnosticReader out of CXLoadedDiagnostic (NFC)"

I'd mispelled "Bitcode/BitCodes.h" before, and tested on a case
insensitive filesystem.

This reverts commit r219649, effectively re-applying r219647 and
r219648.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219664 91177308-0d34-0410-b5e6-96231b3b80d8