Add sequence points on opening/closing braces

[siegfriedpammer]

Currently we only generate sequence points on statements. This makes the debugging experience a bit strange. We should at least add sequence points on the first and last IL offset of a method (mapped to the open/close brace) to prevent a sequence point from "hopping" into the next/previous method when trying to add a sequence point on a brace in VS. In debug builds these should ideally be on nop instructions.

[dgrunwald]

We could do this in debug builds (where there's sufficient nop instructions); but it might be problematic in release builds to find a suitable instruction.
Picking a side-effect-free portion of another instruction (e.g. ldarg.0 for an opening brace) might damage stepping on that instruction if implicit IL sequence points are used instead of the PDB sequence points.
So I don't think this feature would be useful in practice.