[NFC][HLSL] Let RootSignatureParser own the references (#150310)

- this is a clean up from a review comment that we should let the parser
own the constructed `RootSignatureElement`s

Original comment here:
#147115 (comment).

Author: inbelic

clang/include/clang/Parse/ParseHLSLRootSignature.h

@@ -30,7 +30,6 @@ namespace hlsl {
 class RootSignatureParser {
 public:
   RootSignatureParser(llvm::dxbc::RootSignatureVersion Version,
-                      SmallVector<RootSignatureElement> &Elements,
                       StringLiteral *Signature, Preprocessor &PP);
 
   /// Consumes tokens from the Lexer and constructs the in-memory
@@ -40,6 +39,9 @@ class RootSignatureParser {
   /// Returns true if a parsing error is encountered.
   bool parse();
 
+  /// Return all elements that have been parsed.
+  ArrayRef<RootSignatureElement> getElements() { return Elements; }
+
 private:
   DiagnosticsEngine &getDiags() { return PP.getDiagnostics(); }
 
@@ -226,7 +228,7 @@ class RootSignatureParser {
 
 private:
   llvm::dxbc::RootSignatureVersion Version;
-  SmallVector<RootSignatureElement> &Elements;
+  SmallVector<RootSignatureElement> Elements;
   StringLiteral *Signature;
   RootSignatureLexer Lexer;
   Preprocessor &PP;

clang/lib/Parse/ParseDeclCXX.cpp

@@ -4940,17 +4940,16 @@ void Parser::ParseHLSLRootSignatureAttributeArgs(ParsedAttributes &Attrs) {
   // signature string and construct the in-memory elements
   if (!Found) {
     // Invoke the root signature parser to construct the in-memory constructs
-    SmallVector<hlsl::RootSignatureElement> RootElements;
-    hlsl::RootSignatureParser Parser(getLangOpts().HLSLRootSigVer, RootElements,
-                                     Signature, PP);
+    hlsl::RootSignatureParser Parser(getLangOpts().HLSLRootSigVer, Signature,
+                                     PP);
     if (Parser.parse()) {
       T.consumeClose();
       return;
     }
 
     // Construct the declaration.
     Actions.HLSL().ActOnFinishRootSignatureDecl(RootSignatureLoc, DeclIdent,
-                                                RootElements);
+                                                Parser.getElements());
   }
 
   // Create the arg for the ParsedAttr

clang/lib/Parse/ParseHLSLRootSignature.cpp

@@ -27,11 +27,10 @@ static const TokenKind RootElementKeywords[] = {
 };
 
 RootSignatureParser::RootSignatureParser(
-    llvm::dxbc::RootSignatureVersion Version,
-    SmallVector<RootSignatureElement> &Elements, StringLiteral *Signature,
+    llvm::dxbc::RootSignatureVersion Version, StringLiteral *Signature,
     Preprocessor &PP)
-    : Version(Version), Elements(Elements), Signature(Signature),
-      Lexer(Signature->getString()), PP(PP), CurToken(0) {}
+    : Version(Version), Signature(Signature), Lexer(Signature->getString()),
+      PP(PP), CurToken(0) {}
 
 bool RootSignatureParser::parse() {
   // Iterate as many RootSignatureElements as possible, until we hit the