Collect `UnvalidatedDeclsWithOpaqueReturnTypes` in ASTGen

[rintaro]

Motivation

The current C++ parser collects UnvalidatedDeclsWithOpaqueReturnTypes, ASTGen also needs to collect these in order to adopt SwiftParser as the main parser.

Proposed solution

Collect UnvalidatedDeclsWithOpaqueReturnTypes as needed in ASTGen by calling SourceFile::addUnvalidatedDeclWithOpaqueResultType(ValueDecl *) as the current C++ parser does.

Alternatives considered

Implement this as a separate RequestEvaluator request on SourceFile, but UnvalidatedDeclsWithOpaqueReturnTypes is mutable at this point.