Rollup merge of #137643 - beetrees:repr128-dwarf-variant-test, r=jieyouxu

Add DWARF test case for non-C-like `repr128` enums

LLVM 20 fixes DWARF debuginfo for non-C-like 128-bit enums: this PR adds a test case to the `repr128-dwarf` test to ensure that LLVM doesn't regress in the future.

Tracking issue: #56071

Author: compiler-errors