[SyntaxParses] Address memory issue in popTokenSyntax()

Don't return reference for local temporary object

rdar://problem/55421369

Author: rintaro

include/swift/Parse/SyntaxParsingContext.h

@@ -300,7 +300,7 @@ class alignas(1 << SyntaxAlignInBits) SyntaxParsingContext {
     return SyntaxNode(std::move(rawNode));
   }
 
-  ParsedTokenSyntax &&popToken();
+  ParsedTokenSyntax popToken();
 
   /// Create a node using the tail of the collected parts. The number of parts
   /// is automatically determined from \c Kind. Node: limited number of \c Kind

lib/Parse/SyntaxParsingContext.cpp

@@ -163,8 +163,9 @@ const SyntaxParsingContext *SyntaxParsingContext::getRoot() const {
   return Curr;
 }
 
-ParsedTokenSyntax &&SyntaxParsingContext::popToken() {
-  return std::move(popIf<ParsedTokenSyntax>().getValue());
+ParsedTokenSyntax SyntaxParsingContext::popToken() {
+  auto tok = popIf<ParsedTokenSyntax>();
+  return std::move(tok.getValue());
 }
 
 /// Add Token with Trivia to the parts.