[mlir][llvm] Fix export of 64-bit integer function attributes

The `allocsize` attribute is weird because it packs two 32-bit values
into a 64-bit value. It also turns out that the passthrough attribute
exporter was using `int`, which is incorrectly handling 64-bit integers.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D156574