The Protocol field isn't really necessary, because the conformance
stores the protocol. But we do need the substituted subject type
of the requirement, just temporarily, until an abstract conformance
stores its own subject type too.
* let `SIL.Type` conform to `TypeProperties` to share the implementation of common type properties between the AST types and `SIL.Type`
* call references to an `AST.Type` `rawType` (instead of just `type`)
* remove unneeded stuff
* add comments
* factor out common methods of AST Type/CanonicalType into a `TypeProperties` protocol.
* add more APIs to AST Type/CanoncialType.
* move `MetatypeRepresentation` from SIL.Type to AST.Type and implement it with a swift enum.
* let `Builder.createMetatype` get a CanonicalType as instance type, because the instance type must not be a lowered type.
* Collect flag in `ParamDecl::setTypeRepr()`.
* [ASTGen] Separate `BridgedParamDecl.setTypeRepr(_:)` from
`BridgedParamDecl.createParsed(_:)` aligning with C++ API. The majority
of the creations don't set the typerepr.
* Update `ParamSpecifierRequest::evaluate` to handle non-implicit
`ParamDecl` without `TypeRepr` (i.e. untyped closure parameter), instead
of `setSpecifier(::Default)` manually in Parse.
Rework TypeAttribute bridging facilities.
Eliminate the _fake_ `swift::TypeAttributes`, but just use the normal
`BridgedArrayRef` for briding the attribute list. Introduce
`BridgedTypeOrCustomAttr` tagged pointer which resembles
`swift::TypeOrCustomAttr`.
* Instead of hoisting VarDecl in the bridging functions, do it in
ASTGen.
* Introduce `Decl::forEachDeclToHoist` to handle VarDecls in
PatternBindingDecl, and EnumElementDecl in EnumCaseDecl.
* Intorduce `withBridgedSwiftClosure(closure:call:)` as a callback
mechanism between Swift and C++
* In `generate(sourceFile:)`, instead of using `generate(codeBlockItem:)`
handle `CodeBlockItemSyntax.Item` manually to handle `TLCD` wrapping
and `VarDecl` hoisting.
* Make `generate(variableDecl:)` handle TLCD correctly.
* Move `AvailabilitySpec` handling logic to AST, so they can be shared
between libParse and ASTGen
* Requestify '-define-availability' arguments parsing and parse them
with 'SwiftParser' according to the 'ParserASTGen' feature flag
* Implement 'AvailableAttr' generation in ASTGen
And move the implementation of `SIL.Type.canBeClass` to the AST Type. The SIL Type just calls the AST Type implementation.
Also rename `SIL.Type.canonicalASTType` -> `SIL.Type.astType`.
