[mlir][SubElements] Disambiguate cast<> call for MSVC

For some reason MSVC selects Attribute::cast here, and then fails to
call it because it takes no arguments. Feels like a generic lambda compiler
bug.

mlir/include\mlir/IR/StorageUniquerSupport.h(135): error C2672: 'mlir::Attribute::cast': no matching overloaded function found