[flang] Fix symbol on module subroutine name with same name as generic (#67678)

When a MODULE SUBROUTINE or MODULE FUNCTION is implemented in the same
scope as its interface and appears in a generic with the same name, the
parse::Name of the implementation was not correctly reset and remained
the SubprogramNameDetails symbol after semantics, causing a crash in
lowering that picks up the procedure symbols on the parser names.

Reset the parser::Name symbol before the new symbol is created.