Refactor 'TextDiagnostic' to have a parent class 'DiagnosticRenderer' which handles
the policy of how diagnostics are lowered/rendered, while TextDiagnostic handles
the actual pretty-printing.

This is a first part of reworking SerializedDiagnosticPrinter to use the same
inclusion-stack/macro-expansion logic as TextDiagnostic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@146819 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed