Added precomputed breakpoint for IDEs that can auto-resolve ILOffset

Added a new PrecomputedBreakpoint class, inheriting from Breakpoint. The only difference is that Precompute includes method name and ILOffset, which I can find before calling the debugger. This allows me to set breakpoints in delegate code, in the same line, without having to guess location by line/column. I also added a corresponding FindLocationByILOffset, matching the signature of FindLocationByType as much as I could. This change is purely incremental and backwards compatible. If Precomputed is not used, nothing is changed at all.

/cc @jstedfast @migueldeicaza