Accurately annotate Error system with [[noreturn]]

[alexreinking]

This PR splits the ErrorReport type into two types:

1. WarningReport, which does not throw/abort on destruction.
2. ErrorReport, which always throws/aborts on destruction, and is annotated with [[noreturn]].

This makes internal_error a valid statement to use at the end of a function that would otherwise fail to return.

Also applies HALIDE_ALWAYS_INLINE to the operator<< overload because it was getting specialized for every size of const char * literal.

[abadams]

Should CompileTimeErrorReporter::error also be annotated?

[alexreinking]

Should CompileTimeErrorReporter::error also be annotated?

I think so, but I think this will force downstreams to add the annotation, too. Is that ok?

[abadams]

Yes, because it needs to be crystal clear that the method must not return, or there is UB. I suspect the only downstream user is our own python bindings.