[flang] Correctly detect overlapping integer cases

Integer case values were being compared as unsigned by operator<
on evaluate::value::Integer. Change that to signed so that overlap
can be detected correctly.

Explicit CompareUnsigned and BLT are still available if unsigned
comparison is needed.

Fixes https://bugs.llvm.org/show_bug.cgi?id=47309

Differential Revision: https://reviews.llvm.org/D87595