Centralize the management of CXXRecordDecl::DefinitionData's Empty bit

in CXXRecordDecl itself. Yes, this is also part of <rdar://problem/8459981>.

This reinstates r114924, with one crucial bug fix: we were ignoring
the implicit fields created by anonymous structs/unions when updating
the bits in CXXRecordDecl, which means that a class/struct containing
only an anonymous class/struct would be considered "empty". Hilarity
follows. 

llvm-svn: 114980