Revert "Recommit [NFC] Refactor DiagnosticBuilder and PartialDiagnostic"

This reverts commit 8e780a16.

DiagnosticBuilder is a value type, created on the stack everywhere. IMO
we should not be adding a vtable to it, and making very operator<< use a
virtual interface. There are other feasible designs for implementing
this. The original review, D84362, was approved by @tra, who is
responsible for Clang's CUDA support, but it wasn't reviewed by @rsmith
or anyone responsible for clang's diagnostic library.