Fix ICE in decimal_literal_representation lint
Handling the integer parsing properly instead of just unwrapping.
Note that the test is not catching the ICE because plain UI tests
[currently hide ICEs][compiletest_issue]. Once that issue is fixed, this
test would fail properly again.
Fixes#3891
[compiletest_issue]: https://github.com/laumann/compiletest-rs/issues/169
Handling the integer parsing properly instead of just unwrapping.
Note that the test is not catching the ICE because plain UI tests
[currently hide ICEs][compiletest_issue]. Once that issue is fixed, this
test would fail properly again.
[compiletest_issue]: https://github.com/laumann/compiletest-rs/issues/169