Add class-specific operator new to Decl hierarchy. This guarantees that Decls

can't accidentally be allocated the wrong way (missing prefix data for decls
from AST files, for instance) and simplifies the CreateDeserialized functions a
little. An extra DeclContext* parameter to the not-from-AST-file operator new
allows us to ensure that we don't accidentally call the wrong one when
deserializing (when we don't have a DeclContext), allows some extra checks, and
prepares for some planned modules-related changes to Decl allocation.

No functionality change intended.

llvm-svn: 195426