[ASTGen] ParamDecl generation improvements

[rintaro]

DefaultArgumentInitializer decl context for the default argument value.

Modified the flow in C++ parser too. It used to be:

1. Create the initializer context when parsing the expression
2. Remember the created initializers in DefaultArgumentInfo
3. Construct ParamDecl without initContext
4. Create FunctionDecl
5. Set the initContext's parent via DefaultArgumentInitializer::changeFunction
6. Set the ParamDecl.initContext in the same function

This seemed pretty complicated, especially when we can:

1. Create the initializer context when parsing/generating the expression
2. Construct ParamDecl with the initContext
3. Make ParameterList::setDeclContextOfParamDecls set the initContext's parent too.

Generate attributes on ParamDecls

Simple independent change. We didn't generate and set the attributes on function parameters.

[rintaro]

@swift-ci Please smoke test

[hamishknight]

Nice cleanup!

[hamishknight]

Seems like claimNextIndex is unused?

[hamishknight]

Would also be nice to see if we could remove HasDefaultArgument, seems like the diagnostic logic there could either be plumbed down or check the ParameterList itself

[rintaro]

Yeah, DefaultArgumentInfo does mostly nothing anymore. I guess HasDefaultArgument diagnostic can be sinked into TyoeCheckDeclPrimary. I will try it in followups, those diagnostics are needed in ASTGen mode too.

[rintaro]

@swift-ci Pease smoke test

[rintaro]

@swift-ci Please smoke test

[rintaro]

@swift-ci please smoke test Linux

[rintaro]

@swift-ci please smoke test Linux