Instantiate a ClangTidyDiagnosticConsumer also for the plugin case and
let it forward the diagnostics to the external diagnostic engine that is
already in place.
One minor difference to the clang-tidy executable case is that the
compiler checks/diagnostics are referred to with their original name.
For example, for -Wunused-variable the plugin will refer to the check as
"-Wunused-variable" while the clang-tidy executable will refer to that
as "clang-diagnostic- unused-variable". This is because the compiler
diagnostics never reach ClangTidyDiagnosticConsumer.