Fix sig inst (#46886)

Fix sig inst (#46886)

* Fix 32-bit platform handling of CORINFO_SIG_INST

Previous code was reading an array of pointers as an array of
8-byte elements, leading to some cases of reading beyond
allocated memory and causing access violations. Convert remaining
cases to properly serialized/deserialized pointers.

This is a follow-up to #46604.

Moves all Agnostic types to their own file, agnostic.h, so MethodContext
and CompileResult can share helpers with the same types (previously,
they each had a copy of AgnosticCORINFO_SIG_INFO).

* Remove duplicate types

* Convert CompileResult to use common CORINFO_SIG_INFO functions

Add CrSigInstHandleMap to store handle map for CompileResults.

* Change JIT/EE GUID

SuperPMI format has changed, so change the GUID.