[DSLX] Enable conversion from size_type to int64_t to compile in OSS with g++ 11.2.0.
[DSLX] Enable conversion from size_type to int64_t to compile in OSS with g++ 11.2.0.

PiperOrigin-RevId: 415613023

google/xls,google/xls,google/xls,google/xls,google/xls,google/xls