[flang] Don't complain about dummy subroutine under IMPLICIT NONE

The compiler emits a bogus 'No explicit type declared for...' error
when a dummy procedure turns out to be a subroutine (or at least not
a function or object) under control of IMPLICIT NONE.

Fixes https://github.com/llvm/llvm-project/issues/60224

Differential Revision: https://reviews.llvm.org/D150814