There's a lot of stuff wrong with the representation of these types:
TyFnDef doesn't actually uniquely identify a function, TyFnPtr is used to
represent method calls, TyFnDef in the sub-expression of a cast isn't
correctly reified, and probably some other stuff I haven't discovered yet.
Splitting them seems like the right first step, though.

Fixes #30891

rather than being implicit quasi-subtyping. Nothing good can come out
of quasi-subtyping.

The issue was that the const evaluator was returning an error because
the feature flag const_indexing wasn't turned on. The error was then
reported as a bug.

Fixes #29914

The purpose of the translation item collector is to find all monomorphic instances of functions, methods and statics that need to be translated into LLVM IR in order to compile the current crate.
So far these instances have been discovered lazily during the trans path. For incremental compilation we want to know the set of these instances in advance, and that is what the trans::collect module provides.
In the future, incremental and regular translation will be driven by the collector implemented here.

This moves back (essentially reverts #30265) into MIR-specific translation code, but keeps the
funcition split out, since it is expected to eventually become recursive.

+ Apply parser changes manually
+ Add feature gate

tests & rustdoc still broken

Fixes #29578

const eval errors outside of true constant enviroments are not reported anymore, but instead forwarded to a lint.