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.

llvm-svn: 146819