Implement computation of the final overriders for each virtual

Implement computation of the final overriders for each virtual
function within a class hierarchy (C++ [class.virtual]p2).

We use the final-overrider computation to determine when a particular
class is ill-formed because it has multiple final overriders for a
given virtual function (e.g., because two virtual functions override
the same virtual function in the same virtual base class). Fixes
PR5973.

We also use the final-overrider computation to determine which virtual
member functions are pure when determining whether a class is
abstract or diagnosing the improper use of an abstract class. The
prior approach to determining whether there were any pure virtual
functions in a class didn't cope with virtual base class subobjects
properly, and could not easily be fixed to deal with the oddities of
subobject hiding. Fixes PR6631.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99351 91177308-0d34-0410-b5e6-96231b3b80d8