[DSLX] `assert!` and `fail!` labels can clash in generated verilog (leading to e.g. elab warnings)

[cdleary]

Describe the bug
If you use the same label for a fail! and an assert! they can clash as asserted property labels in the generated sv, which can lead to things like elab warnings, where we probably want to give an early error.

To Reproduce
assert!(something, "my_name");
fail!("my_name", default_value);

Expected behavior
Probably flagging that the label is being reused in the function scope.