When using libc++, Swift imports `size_t` as Int despite `size_t` being an unsigned type. This is intentional & is specified in `lib/ClangImporter/MappedTypes.def`. Previously, MappedTypes were only honored for C/C++ types declared on the file level.
In libstdc++, `size_t` is declared within `namespace std` and not on the file level, so the mapping to Int was not applied.
This change ensures that MappedTypes are also applied to types declared in `namespace std`.
