[flang] Fixes for problems with declaring procedure entities

Summary:
We were not detecting declaring multiple interfaces to the same procedure.
Also, we were not handling the initialization of entitiies where the associated
Symbol had previously had errors.

I added the function `IsInterfaceSet()` to ProcEntityDetails to see if
the value of `interface_` had been previously set.  I then checked  this
function before calling set_interface() and emitted an error message if
the interface was already set.

Also, in situations where we were emitting error messages for symbols, I
set the Error flag on the Symbol.  Then when performing initialization
on the Symbol, I first check to see if the Symbol had an error.

Reviewers: tskeith, klausler, DavidTruby

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80453