Move the maintenance of CXXRecordDecl::DefinitionData's Abstract bit

completely into CXXRecordDecl, by adding a new completeDefinition()
function. This required a little reshuffling of the final-overrider
checking code, since the "abstract" calculation in the presence of
abstract base classes needs to occur in
CXXRecordDecl::completeDefinition() but we don't want to compute final
overriders more than one in the common case.

llvm-svn: 115007