[Parser] Gate 'repeat' parsing for variadic generics behind the experimental

feature flag to preserve existing parser diagnostics until the feature is
official.

Author: hborla

lib/Parse/ParseExpr.cpp

@@ -512,7 +512,8 @@ ParserResult<Expr> Parser::parseExprUnary(Diag<> Message, bool isExprBasic) {
   tryLexRegexLiteral(/*forUnappliedOperator*/ false);
 
   // 'repeat' as an expression prefix is a pack expansion expression.
-  if (Tok.is(tok::kw_repeat)) {
+  if (Context.LangOpts.hasFeature(Feature::VariadicGenerics) &&
+      Tok.is(tok::kw_repeat)) {
     SourceLoc repeatLoc = consumeToken();
     auto patternExpr = parseExpr(Message);
     if (patternExpr.isNull())

lib/Parse/ParseStmt.cpp

@@ -66,7 +66,8 @@ bool Parser::isStartOfStmt() {
     // is a pack expansion expression.
     // FIXME: 'repeat' followed by '{' could be a pack expansion
     // with a closure pattern.
-    return peekToken().is(tok::l_brace);
+    return !Context.LangOpts.hasFeature(Feature::VariadicGenerics) ||
+           peekToken().is(tok::l_brace);
 
   case tok::pound_line:
     // #line at the start of a line is a directive, when within, it is an expr.