[AsmParser][SystemZ][z/OS] Introducing HLASM Comment Syntax

[AsmParser][SystemZ][z/OS] Introducing HLASM Comment Syntax

- This patch adds in support for the ordinary HLASM comment syntax asm
  statements (Reference - Chapter 7, Comment Statements, Ordinary Comment
  Statements)
- In brief, the ordinary comment syntax if used, must begin with the "*"
  character
- To achieve this, this patch makes use of the CommentString attribute
  provided in the base MCAsmInfo class
- In the SystemZMCAsmInfo class, the CommentString attribute was set to
  "*" based on the assembler dialect
- Furthermore, a new attribute RestrictCommentString, is provided to only
  treat a string as a comment if it appears at the start of the asm
  statement. Example: "jo *-4" is valid in HLASM (jump back 4 bytes from
  current point - similar to jo -4 in gnu asm) and we don't want "*-4" to
  be treated as a comment.
- RFC for HLASM Parser support implementation: https://lists.llvm.org/pipermail/llvm-dev/2021-January/147686.html

Reviewed By: scott.linder, Kai

Differential Revision: https://reviews.llvm.org/D97703