LTO: Ignore disabled diagnostic remarks

r206400 and r209442 added remarks that are disabled by default.
However, if a diagnostic handler is registered, the remarks are sent
unfiltered to the handler.  This is the right behaviour for clang, since
it has its own filters.

However, the diagnostic handler exposed in the LTO API receives only the
severity and message.  It doesn't have the information to filter by pass
name.  For LTO, disabled remarks should be filtered by the producer.

I've changed `LLVMContext::setDiagnosticHandler()` to take a `bool`
argument indicating whether to respect the built-in filters.  This
defaults to `false`, so other consumers don't have a behaviour change,
but `LTOCodeGenerator::setDiagnosticHandler()` sets it to `true`.

To make this behaviour testable, I added a `-use-diagnostic-handler`
command-line option to `llvm-lto`.

This fixes PR21108.