[llvm] [LLVM][AArch64] Build attributes: Support switching to a defined subsection by name only (PR #154159)

[Peter Smith via llvm-commits]

================
@@ -7925,7 +7927,25 @@ bool AArch64AsmParser::parseDirectiveAeabiSubSectionHeader(SMLoc L) {
     return true;
   }
   Parser.Lex();
-  // consume a comma
+
+  // Check whether only the subsection name was provided.
----------------
smithp35 wrote:

I think you can simplify the change by moving the call to SubsectionExists here. This can supply the parameters for `emitAttributesSubsection` so you wouldn't need to change anything in the `AArch64TargetStreamer`. 
```
  std::unique_ptr<MCELFStreamer::AttributeSubSection> SubsectionExists =
      getTargetStreamer().getAttributesSubsectionByName(SubsectionName);

  if (Parser.getTok().is(llvm::AsmToken::EndOfStatement)) {
    if (SubsectionExists) {
      getTargetStreamer().emitAttributesSubsection(SubsectionName, SubsectionExists->IsOptional, SubsectionExists->Type);
      return false;
    }
    else {
      Error(Parser.getTok().getLoc(),
            "Could not switch to subsection '" + SubsectionName +
                "' using subsection name, subsection has not been defined");
      return true;
    }
  }
  // Otherwise, expecting 2 more parameters: consume a comma
```

https://github.com/llvm/llvm-project/pull/154159