[SystemZ][z/OS] Enable the AllowAtInName attribute for the HLASM dialect

[anirudhp]

Summary

• Currently, LLVM supports symbols of the name "token1@token2".
• "token2" is used to identify whether an appropriate symbol reference can be used for the symbol.
• Now, if the symbol reference couldn't be found, the AsmParser usually emits an error, unless the backend is configured to accept the "@" in a symbol name
• Thus, this patch aims to do that. It sets the AllowAtInName attribute in the SystemZ backend for the HLASM dialect.
• Setting this attribute ensures that, if a particular symbol reference is found, it uses that. If it doesn't, and there exists an "@" in the symbol name, it will use that instead of explicitly erroring out.

[uweigand]

LGTM.

llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmInfo.cpp

SystemZMCAsmInfo::SystemZMCAsmInfo(const Triple &TT) {
   RestrictCommentStringToStartOfStatement = (AssemblerDialect == AD_HLASM);
   AllowAdditionalComments = (AssemblerDialect == AD_ATT);
   AllowAtAtStartOfIdentifier = (AssemblerDialect == AD_HLASM);
   AllowDollarAtStartOfIdentifier = (AssemblerDialect == AD_HLASM);
   AllowHashAtStartOfIdentifier = (AssemblerDialect == AD_HLASM);
   DotIsPC = (AssemblerDialect == AD_ATT);
   StarIsPC = (AssemblerDialect == AD_HLASM);
   EmitGNUAsmStartIndentationMarker = (AssemblerDialect == AD_ATT);
+  AllowAtInName = (AssemblerDialect == AD_HLASM);
 
   ZeroDirective = "\t.space\t";
   Data64bitsDirective = "\t.quad\t";
   UsesELFSectionDirectiveForBSS = true;
   SupportsDebugInformation = true;
   ExceptionsType = ExceptionHandling::DwarfCFI;
 }
 
 bool SystemZMCAsmInfo::isAcceptableChar(char C) const {
   if (AssemblerDialect == AD_ATT)
     return MCAsmInfo::isAcceptableChar(C);
 
   return MCAsmInfo::isAcceptableChar(C) || C == '#';
 }