Fix clang debug info irgen of i128 enums

DIEnumerator stores an APInt as of April 2020, so now we don't need to
truncate the enumerator value to 64 bits. Fixes assertions during IRGen.

Split from D105320, thanks to Matheus Izvekov for the test case and
report.

Differential Revision: https://reviews.llvm.org/D106585