[flang][OpenMP] Avoid marking named main programs as declare target (#190250)
A bare `!$omp declare target` could incorrectly mark `_QQmain` as
`omp.declare_target` when it appeared in an interface body inside a
named
main program. That pulled host-only callees into device compilation and
caused offload link failures.
Fix this by skipping main programs in the implicit-capture path.
Also add a regression test for the named-main interface case and update
`real10.f90` to use a valid container for the bare `declare target`
form.
This fixes offload link failures where `_QQmain` was incorrectly treated
as
a device function and pulled in host-only symbols such as Fortran I/O
runtime calls.
Minimal reproducer:
```fortran
program named_main
interface
subroutine sub_a(x)
!$omp declare target
integer, intent(inout) :: x
end subroutine
end interface
integer :: v
!$omp target
call sub_a(v)
!$omp end target
end program
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