[flang] Complement one-to-one association check of bind name and entity name

As Fortran 2018 C802 and C873, if bind name is specified, there can only
be only one entity. The check for common block is missed before. As
Fortran 2018 8.5.5 point 2, the bind name is one identifier, which is
unique. That is, one entity can not have multiple bind names. Also add
this check.

Reviewed By: klausler, Jean Perier

Differential Revision: https://reviews.llvm.org/D126961