We insert end_cow_mutation_addr for lifetime dependent values dependent on mutable addresses.
end_cow_mutation_addr can be simplified to end_cow_mutation after other optimizations like inlining, specialization etc
This PR adds an instruction simplification to transform end_cow_mutation_addr to end_cow_mutation.
This can enable array optimizations which look for end_cow_mutation.
As the optimizer uses more and more AST stuff, it's now time to create an "AST" module.
Initially it defines following AST datastructures:
* declarations: `Decl` + derived classes
* `Conformance`
* `SubstitutionMap`
* `Type` and `CanonicalType`
Some of those were already defined in the SIL module and are now moved to the AST module.
This change also cleans up a few things:
* proper definition of `NominalTypeDecl`-related APIs in `SIL.Type`
* rename `ProtocolConformance` to `Conformance`
* use `AST.Type`/`AST.CanonicalType` instead of `BridgedASTType` in SIL and the Optimizer
