Parse expanded buffer into dedicated syntax.
Also rename `BridgedGeneratedSourceFileKindAttribute` to
`BridgedGeneratedSourceFileKindAttributeFromClang` because C++ decl
(i.e. `GeneratedSourceInfo::Kind::AttributeFromClang`) was renamed a
while ago.
* 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.
* 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.
Use `Decl::attachParsedAttrs()` instead of `Decl::setAttrs()` to attach attributes to a declaration in ASTGen. This causes the common attribute-setup logic there to be run.
NFC in this commit because none of the attributes that have special setup logic are currently implemented in ASTGen. Prepares to add support for `@abi` in a future commit.
