Nothing in the affected code depends on the `ModuleName` being
null-terminated,
so take it by `StringRef` instead of `const std::string &`.
This change simplifies API consumption, since one doesn't always have a
`std::string` at the call site (might have `std::string_view` instead),
and also gives some minor performance improvements by removing
string-copies in the cache-hit path of `getOrCreateModuleInfo`.
bf88db78bd