[SystemZ][z/OS] Add support to validate a HLASM Label.

- This patch adds in support to determine whether a particular label
  is valid for the hlasm variant
- The label syntax being checked is that of an ordinary HLASM symbol
  (Reference, Chapter 2 (Coding and Structure) - Terms, Literals and
  Expressions - Terms - Symbols - Ordinary Symbol)
- To achieve this, the virtual function isLabel defined in
  MCTargetAsmParser.h is made use of
- The isLabel function is overridden in SystemZAsmParser for the
  hlasm variant, and the syntax is checked appropriately
- Things remain unchanged for the att variant
- Further patches will add in support to emit the label. These future
  patches will make use of this isLabel function

Reviewed By: uweigand, Kai

Differential Revision: https://reviews.llvm.org/D97748

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