More string fixes for noexcept cases. Apparently I didn't get them all in r258281.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@258291 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.modifiers/string_append/iterator.pass.cpp

@@ -27,6 +27,7 @@ test(S s, It first, It last, S expected)
     assert(s == expected);
 }
 
+#ifndef TEST_HAS_NO_EXCEPTIONS
 template <class S, class It>
 void
 test_exceptions(S s, It first, It last)
@@ -40,6 +41,7 @@ test_exceptions(S s, It first, It last)
     assert(s.__invariants());
     assert(s == aCopy);
 }
+#endif
 
 int main()
 {