Parse: repair build after #26981
The build on Windows has been failing due to the ambiguous constructor
overload:
```
lib\Parse\ParseRequests.cpp(83): error C2666: 'swift::Parser::Parser': 2 overloads have similar conversions
include\swift\Parse\Parser.h(412): note: could be 'swift::Parser::Parser(std::unique_ptr<swift::Lexer,std::default_delete<_Ty>>,swift::SourceFile &,swift::SILParserTUStateBase *,swift::PersistentParserState *,std::shared_ptr<swift::SyntaxParseActions>,bool)'
with
[
_Ty=swift::Lexer
]
include\swift\Parse\Parser.h(408): note: or 'swift::Parser::Parser(unsigned int,swift::SourceFile &,swift::SILParserTUStateBase *,swift::PersistentParserState *,std::shared_ptr<swift::SyntaxParseActions>,bool)'
include\swift\Parse\Parser.h(403): note: or 'swift::Parser::Parser(unsigned int,swift::SourceFile &,swift::DiagnosticEngine *,swift::SILParserTUStateBase *,swift::PersistentParserState *,std::shared_ptr<swift::SyntaxParseActions>,bool)'
lib\Parse\ParseRequests.cpp(84): note: while trying to match the argument list '(unsigned int, swift::SourceFile, nullptr, nullptr, nullptr, bool)'
```
