[flang] Better error handling and testing of generics with homonymous specifics or derived types

Fortran allows a generic procedure interface to have the same name as a derived
type in the same scope or the same name as one of its specific procedures.
(It can't have both since a derived type and specific procedure can't have the
same name in a scope.)

Some popular compilers allow generic interfaces with distinct accessible homonymous
specific procedures to be merged by USE association.  Thsi compiler does not,
and for good reason: it leads to ambiguity in cases where a procedure name appears
outside a reference, such as in a PROCEDURE declaration statement as the procedure's
interface, the target of a procedure pointer assignment statement, or as an
actual argument.

This patch cleans up the code that handles these cases, improves some error
messages, and adds more tests.

Resolves https://github.com/llvm/llvm-project/issues/60228.

Differential Revision: https://reviews.llvm.org/D150915