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
The buffer of global arrays could already be statically initialized.
The missing piece was the array itself, which is basically a reference to the array buffer.
For example:
```
var a = [1, 2, 3]
```
ends up in two statically initialized globals:
1. the array buffer, which contains the elements
2. the variable `a` which is a single reference (= pointer) of the array buffer
This optimization removes the need for lazy initialization of such variables.
rdar://127757554
* add `NominalTypeDecl.isResilient`
* make the return type of `Type.getNominalFields` optional and return nil in case the nominal type is resilient.
This forces users of this API to think about what to do in case the nominal type is resilient.
A transformation must not create a `struct` instruction with a non-copyable type because this would apply that a (potential) deinit would be called for that struct.
When merging stores in a global initializer, it's possible that the merged store is inserted at the wrong location, causing a SIL verifier error.
This is hard to reproduce, but can happen.
The merged store must be inserted _after_ all other stores. Instead it's inserted after the store of the last property. Now, if properties are _not_ initialized in the order they are declared, this problem can show up.
rdar://117189962
For example:
```
var p = Point(x: 10, y: 20)
let o = UnsafePointer(&p)
```
Also support outlined arrays with pointers to other globals. For example:
```
var g1 = 1
var g2 = 2
func f() -> [UnsafePointer<Int>] {
return [UnsafePointer(&g1), UnsafePointer(&g2)]
}
```
Sometimes structs are not stored in one piece, but as individual elements. Merge such individual stores to a single store of the whole struct.
This enables generating a statically initialized global.
* add the StaticInitCloner utility
* remove bridging of `copyStaticInitializer` and `createStaticInitializer`
* add `Context.mangleOutlinedVariable` and `Context.createGlobalVariable`
It converts a lazily initialized global to a statically initialized global variable.
When this pass runs on a global initializer `[global_init_once_fn]` it tries to create a static initializer for the initialized global.
```
sil [global_init_once_fn] @globalinit {
alloc_global @the_global
%a = global_addr @the_global
%i = some_const_initializer_insts
store %i to %a
}
```
The pass creates a static initializer for the global:
```
sil_global @the_global = {
%initval = some_const_initializer_insts
}
```
and removes the allocation and store instructions from the initializer function:
```
sil [global_init_once_fn] @globalinit {
%a = global_addr @the_global
%i = some_const_initializer_insts
}
```
The initializer then becomes a side-effect free function which let's the builtin-simplification remove the `builtin "once"` which calls the initializer.
