[clang] [Clang][Lex] Fix parsing of nested requirement to prevent flowing off the end of token stream (PR #73691)

[Shafik Yaghmour via cfe-commits]

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@@ -3639,6 +3639,10 @@ ExprResult Parser::ParseRequiresExpression() {
                 //  TryParseParameterDeclarationClause).
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shafik wrote:

I don't think the above fixme above is needed, I don't think there is any other way to handle this, at least based on some of the other places `TryParseParameterDeclarationClause()` is used e.g. `TryParseFunctionDeclarator(...)`

https://github.com/llvm/llvm-project/pull/73691