[flang] Specific procedures named the same as the generic and a derived type

If you specify a specific procedure of a generic interface that has the same
name as both the generic interface and a preceding derived type, the compiler
would fail an internal call to CHECK().  I fixed this by testing for this
situation when processing specific procedures.  I also added a test that will
cause the call to CHECK() to fail without this new code.

Differential Revision: https://reviews.llvm.org/D99085