[SystemZ][z/OS] Introduce dialect querying helper functions

- In the SystemZAsmParser, there will be a few queries to the type of dialect it is (AD_ATT, AD_HLASM) in future patches.
- It would be nice to have two small helper functions `isParsingATT()` and `isParsingHLASM()`
- Putting this as a separate smaller patch allows us to remove its definitions from other dependent patches.

Reviewed By: uweigand, abhina.sreeskantharajan

Differential Revision: https://reviews.llvm.org/D99891