CFI: Support function pointers for trait methods
Adds support for both CFI and KCFI for function pointers to trait methods by attaching both concrete and abstract types to functions.
KCFI does this through generation of a `ReifyShim` on any function pointer for a method that could go into a vtable, and keeping this separate from `ReifyShim`s that are *intended* for vtable us by setting a `ReifyReason` on them.
CFI does this by setting both the concrete and abstract type on every instance.
This should land after #123024 or a similar PR, as it diverges the implementation of CFI vs KCFI.
r? `@compiler-errors`
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