Reapply "[flang] Improve debug info for functions." with regression fixed. (#90484)

The original PR #90083 had to be reverted in PR #90444 as it caused one
of the gfortran tests to fail. The issue was using `isIntOrIndex` for
checking for integer type. It allowed index type which later caused
assertion when calling `getIntOrFloatBitWidth`. I have now replaced it
with `isInteger` which should fix this regression.