[clang][Modules] Make `Module::Requirement` a struct (NFC) (#67900)

`Module::Requirement` was defined as a `std::pair<std::string, bool>`.
This required a comment to explain what the data members mean and makes
the usage harder to understand. Replace this with a struct with two
members, `FeatureName` and `RequiredState`.

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Co-authored-by: cor3ntin <corentinjabot@gmail.com>