Add pretty-print capability

[d-w-moore]

Replacing Clang 6's libcxx and libcxx-api shared objects and installing these pretty-printers
could make debugging a bit easier by outputting this for a map<string,string>:

$1 = std::__1::map (count=2) = {[0] "key1" = "value1", [1] "key2" = "value

instead of this:

(gdb) p /r mymap
$2 = {__tree_ = {__begin_node_ = 0x608010,
  __pair1_ = {<std::__1::__compressed_pair_elem<std::__1::__tree_end_node<std::__1::__tree_node_base<void*>*>, 0, false>> = {__value_ = {         __left_ = 0x608010}}, <std::__1::__compressed_pair_elem<std::__1::allocator<std::__1::__tree_node<std::__1::__value_type<std::__1::basic_string<char>, ...
          ( ... lines of internal map and string implementation detail we don't want to see...)
  <No data fields>}, <No data fields>}}}

[d-w-moore]

addressed here