Tolerate missing debug info in the shared_ptr pretty printer.

Certain fields of shared ptr have virtual functions and therefore
have their debug info homed in libc++. But if libc++ wasn't built
with debug info, the pretty printer would fail.

This patch makes the pretty printer tolerate such conditions and
updates the test harness.

This patch significantly reworks a previous attempt.

This addresses https://bugs.llvm.org/show_bug.cgi?id=48937

Differential Revision: https://reviews.llvm.org/D100610

(cherry picked from commit 55b70611)