Remove all uses of DIFlagBlockByrefStruct

This patch removes the last reason why DIFlagBlockByrefStruct from
Clang by directly implementing the drilling into the member type done
in DwarfDebug::DbgVariable::getType() into the frontend.

rdar://problem/31629055

Differential Revision: https://reviews.llvm.org/D51807

llvm-svn: 341842