Add gdb pretty printers for a wide variety of libc++ data structures (take 2).

Summary:
This patch is an exact duplicate of https://reviews.llvm.org/D65609, except
that it uses the newly introduced testing framework to detect if gdb is present
so that the tests won't fail on machines without gdb.

Reviewers: echristo, EricWF

Subscribers: christof, ldionne, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67238

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@371131 91177308-0d34-0410-b5e6-96231b3b80d8