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

**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.