For example, this covers the following:
```
template <typename T>
    requires std::integral<T>
int foo();
```

Refactor parsing of `Q <requires-clause expr>` into a single function that performs backtracking to avoid reimplementing `ParseTemplateArgs` in terms of nested if-else blocks.

PiperOrigin-RevId: 605785418
Change-Id: I118998a75e050dcf46af125b613b690312fd3cbe