Fix CSApply bug for keyword-based dynamic callable. (#24580)

The type set by cs.setType() in the dynamic callable application did not
have type-variables, so it was considering this string literal already
checked. This made it to SILGen with a nullptr keyword argument.

Author: pschuh

lib/Sema/CSApply.cpp

@@ -2066,10 +2066,11 @@ namespace {
     }
 
     Expr *handleStringLiteralExpr(LiteralExpr *expr) {
-      if (cs.getType(expr) && !cs.getType(expr)->hasTypeVariable())
+      auto stringLiteral = dyn_cast<StringLiteralExpr>(expr);
+      if (cs.getType(expr) && !cs.getType(expr)->hasTypeVariable()
+          && stringLiteral->getBuiltinInitializer())
         return expr;
 
-      auto stringLiteral = dyn_cast<StringLiteralExpr>(expr);
       auto magicLiteral = dyn_cast<MagicIdentifierLiteralExpr>(expr);
       assert(bool(stringLiteral) != bool(magicLiteral) &&
              "literal must be either a string literal or a magic literal");