JIT: pgo/devirt diagnostic improvements

[AndyAyersMS]

Several changes to help better diagnose PGO and devirtualization issues:

• Report the source of the PGO data to the jit
• Report the reason for a devirtualization failure to the jit
• Add checking mode that compares result of devirtualzation to class profile
• Add reporting mode to assess overall rates of devirtualization failure
  when the jit has exact type information.

Also fix a loophole where in some case we'd still devirtualize if not
optimizing.

Note crossgen2 does not yet set devirtualization failure reasons.

[AndyAyersMS]

cc @dotnet/jit-contrib @davidwrighton

A lot of files touched as I've modified 2 apis on the jit interface. No diffs (per PMI).

[davidwrighton]

Please describe the memory behavior of this pointer. In particular, is it expected to be remain across multiple calls to resolveVirtulaMethod?

Also, superpmi is not currently storing/restoring this value.

[AndyAyersMS]

Currently it is only used immediately after the call.

I suppose it would be simpler to make it an enum. I made it a string because I anticipated there would be a lot of churn as we evolve this and didn't want to also have to rev the jit interface each time.

I don't expect actual behavior of the jit to depend on this -- it is just there for diagnostics -- so perhaps an enum with an understanding that a given jit build might see values outside its known enum range will work.

[davidwrighton]

A string is a fine choice. I just want to document its memory behavior, and used directly after the call is a simple and fairly common behavior in the jit interface. The only concern is that handling of strings across the managed/native boundary needs to have some care taken around object lifetime, etc, and writing down the contract is necessary.

[AndyAyersMS]

I went ahead and changed it over to an enum, it's simple for SPMI to deal with.

[AndyAyersMS]

Running Pri1 tests with

set COMPlus_ReadyToRun=0
set COMPlus_TieredCompilation=0
set COMPlus_JitNoteFailedExactDevirtualization=1

gives the following exact class devirt failure breakdown:

reason	count
object class could not be cast to interface class	2799
virtual method installed via explicit override	158
devirtualization crossed version bubble	36
interface method was default interface method	7
interface method could not be found	6

Many of the failures in of that first category are cases where we have a shared interface method.