Clean up the handling of the DeclaredDefaultConstructor and

DeclaredCopyConstructor bits in CXXRecordDecl's DefinitionData
structure. Rather than having Sema call addedConstructor or set the
bits directly at semi-random places, move all of the logic for
managing these bits into CXXRecordDecl itself and tie the
addedConstructor call into DeclContext::addDecl().

This makes it easier for AST-building clients to get the right bits
set in DefinitionData, and is one small part of <rdar://problem/8459981>.

llvm-svn: 114889