[SyntaxParse] Fix ASAN issue

rdar://problem/55711699
rdar://problem/55711787
rdar://problem/55711952

Author: rintaro

Diff for: lib/Parse/ASTGen.cpp

@@ -574,8 +574,11 @@ GenericParamList *ASTGen::generate(const GenericParameterClauseSyntax &clause,
 
     DeclAttributes attrs;
     if (auto attrsSyntax = elem.getAttributes()) {
-      auto attrsLoc = advanceLocBegin(Loc, *attrsSyntax->getFirstToken());
-      attrs = getDeclAttributes(attrsLoc);
+      if (auto firstTok = attrsSyntax->getFirstToken()) {
+        auto attrsLoc = advanceLocBegin(Loc, *firstTok);
+        if (hasDeclAttributes(attrsLoc))
+          attrs = getDeclAttributes(attrsLoc);
+      }
     }
     Identifier name = Context.getIdentifier(elem.getName().getIdentifierText());
     SourceLoc nameLoc = advanceLocBegin(Loc, elem.getName());

Diff for: lib/Parse/ParseGeneric.cpp

@@ -73,12 +73,17 @@ Parser::parseGenericParameterClauseSyntax() {
 
     // Parse attributes.
     // TODO: Implement syntax attribute parsing.
-    DeclAttributes attrsAST;
-    parseDeclAttributeList(attrsAST);
-    auto attrs = SyntaxContext->popIf<ParsedAttributeListSyntax>();
-    if (attrs) {
-      paramBuilder.useAttributes(std::move(*attrs));
-      Generator.addDeclAttributes(attrsAST, attrsAST.getStartLoc());
+    if (Tok.is(tok::at_sign)) {
+      SyntaxParsingContext TmpCtxt(SyntaxContext);
+      TmpCtxt.setTransparent();
+
+      DeclAttributes attrsAST;
+      parseDeclAttributeList(attrsAST);
+      if (!attrsAST.isEmpty())
+        Generator.addDeclAttributes(attrsAST, attrsAST.getStartLoc());
+      auto attrs = SyntaxContext->popIf<ParsedAttributeListSyntax>();
+      if (attrs)
+        paramBuilder.useAttributes(std::move(*attrs));
     }
 
     // Parse the name of the parameter.