[AST] Sink the flag collection logic from Parse to AST
* 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.
