[clang] [llvm] [NFC][RootSignatures] Conform to new std::optional calling conventions (PR #136747)

[Damyan Pepper via cfe-commits]

================
@@ -24,100 +26,222 @@ RootSignatureParser::RootSignatureParser(SmallVector<RootElement> &Elements,
 
 bool RootSignatureParser::parse() {
   // Iterate as many RootElements as possible
-  while (tryConsumeExpectedToken(TokenKind::kw_DescriptorTable)) {
-    // Dispatch onto parser method.
-    // We guard against the unreachable here as we just ensured that CurToken
-    // will be one of the kinds in the while condition
-    switch (CurToken.TokKind) {
-    case TokenKind::kw_DescriptorTable:
-      if (parseDescriptorTable())
+  do {
+    if (tryConsumeExpectedToken(TokenKind::kw_DescriptorTable)) {
+      auto Table = parseDescriptorTable();
+      if (!Table.has_value())
         return true;
-      break;
-    default:
-      llvm_unreachable("Switch for consumed token was not provided");
+      Elements.push_back(*Table);
     }
+  } while (tryConsumeExpectedToken(TokenKind::pu_comma));
 
-    if (!tryConsumeExpectedToken(TokenKind::pu_comma))
-      break;
-  }
-
-  if (!tryConsumeExpectedToken(TokenKind::end_of_stream)) {
-    getDiags().Report(CurToken.TokLoc, diag::err_hlsl_unexpected_end_of_params)
-        << /*expected=*/TokenKind::end_of_stream
-        << /*param of=*/TokenKind::kw_RootSignature;
+  if (consumeExpectedToken(TokenKind::end_of_stream,
+                           diag::err_hlsl_unexpected_end_of_params,
+                           /*param of=*/TokenKind::kw_RootSignature))
----------------
damyanp wrote:

```suggestion
  return consumeExpectedToken(TokenKind::end_of_stream,
                           diag::err_hlsl_unexpected_end_of_params,
                           /*param of=*/TokenKind::kw_RootSignature);
```

https://github.com/llvm/llvm-project/pull/136747