Port llvm r342166 to libcxxabi demangler

Summary:
This was committed back in september (D51463), but it seems it never
made it into the libcxxabi copy.

The original commit message was:
  The hash computed for an ArrayType was different when first constructed
  versus when later profiled due to the constructor default argument, and
  we were not tracking constructor / destructor variant as part of the
  mangled name AST, leading to incorrect equivalences.

Reviewers: erik.pilkington, rsmith, EricWF

Subscribers: christof, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D53063

llvm-svn: 344121