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

[Ashley Coleman via cfe-commits]

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@@ -26,22 +26,14 @@ 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);
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V-FEXrt wrote:

what does this line map to in the old code? I'm not seeing it anywhere

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