c++: Avoid ICE when constant evaluation of __builtin_strchr fails.

If we can't change the argument to &"...", use the original arg instead of
the partially munged one.

	PR c++/93331 - ICE with __builtin_strchr.
	* constexpr.c (cxx_eval_builtin_function_call): Use the original
	argument if we didn't manage to extract a STRING_CST.

gcc/cp/ChangeLog

@@ -1,5 +1,9 @@
 2020-01-23  Jason Merrill  <jason@redhat.com>
 
+	PR c++/93331 - ICE with __builtin_strchr.
+	* constexpr.c (cxx_eval_builtin_function_call): Use the original
+	argument if we didn't manage to extract a STRING_CST.
+
 	PR c++/93345 - ICE with defaulted dtor and template.
 	PR c++/33799
 	* decl.c (cxx_maybe_build_cleanup): Don't try to set

gcc/cp/constexpr.c

@@ -1293,6 +1293,7 @@ cxx_eval_builtin_function_call (const constexpr_ctx *ctx, tree t, tree fun,
   for (i = 0; i < nargs; ++i)
     {
       tree arg = CALL_EXPR_ARG (t, i);
+      tree oarg = arg;
 
       /* To handle string built-ins we need to pass ADDR_EXPR<STRING_CST> since
 	 expand_builtin doesn't know how to look in the values table.  */
@@ -1327,6 +1328,8 @@ cxx_eval_builtin_function_call (const constexpr_ctx *ctx, tree t, tree fun,
 	    arg = braced_lists_to_strings (TREE_TYPE (arg), arg);
 	  if (TREE_CODE (arg) == STRING_CST)
 	    arg = build_address (arg);
+	  else
+	    arg = oarg;
 	}
 
       args[i] = arg;