[AsmParser][SystemZ][z/OS] Fix hanging scenario in HLASMAsmParser class

[anirudhp]

Summary

• In the caller of the overridden parseStatement function (i.e. the AsmParser::Run()) in the case of an error and if we're not at the start of the statement, we "eat" up until the end of the current statement, so we don't have to process it again.
• However, in the HLASMAsmParser class what's happening is that, if an error occurs at the very start of the statement (for example, you invoke the HLASMAsmParser to parse a gnu directive), we will error out, but we never really progress in terms of the next token in the statement to parse. We simply keep looping processing the same error over and over again (partly because we're at the start of the statement)
• To remedy this, when the parseAsHLASMLabel function fails, before returning, we "eat" until the end of the statement function, so we don't process it anymore.

[uweigand]

LGTM.

llvm/lib/MC/MCParser/AsmParser.cpp

if (Lexer.is(AsmToken::EndOfStatement)) {
     }
   }
 
   // Handle the label first if we have to before processing the rest
   // of the tokens as a machine instruction.
   if (ShouldParseAsHLASMLabel) {
     // If there were any errors while handling and emitting the label,
     // early return.
-    if (parseAsHLASMLabel(Info, SI))
+    if (parseAsHLASMLabel(Info, SI)) {
+      // If we know we've failed in parsing, simply eat until end of the
+      // statement. This ensures that we don't process any other statements.
+      eatToEndOfStatement();
       return true;
     }
+  }
 
   return parseAsMachineInstruction(Info, SI);
 }
 
 namespace llvm {
 namespace MCParserUtils {
 
 /// Returns whether the given symbol is used anywhere in the given expression,