There are two piece of codes in ASTWriterDecl to decide whether or not
we should generate a function or a variable in current module unit (or
PCH with object file extension, which is rarely used). One is in
Visit*Decl and One is in `CanElideDef`. Since they are similar it should
be better to merge them.
This was meant to be a NFC patch. But it seems it helped me to find an
existing bug.