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

[Ashley Coleman via cfe-commits]

================
@@ -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())
----------------
V-FEXrt wrote:

nit: `std::optional` should already implement `operator bool()` 
```suggestion
      if (!Table)
```

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