Added hack when serializing DeclRefExprs. This should probably be fixed.

Some FunctionDecls do not appear at the top-level or are owned by a DeclStmt.
In calls to implicitly defined functions, a FunctionDecl is created, but only
the DeclRefExprs reference them. Since an implicitly defined function may be
called multiple times, there is no clear ownership model for such objects.

Temporary solution: when serializing out DeclRefExprs, emit an ownership bit
for the Decl. This bit is determined by querying the serializer to see if the
Decl has already been serialized. If it hasn't, emit the Decl as an owned
pointer.

I repeat: this is a hack.  This should be fixed.

llvm-svn: 44176