[flang] Warn about violations of an obscure requirement (15.6.4 p2)

The Fortran 2018 standard, perhaps as an attempt to prevent ambiguity
in  older compilers, requires that a statement function appear in an
explicit type declaration statement if its name is also accessible
from a host scope.  F18 processes the specification parts of inner
procedures first, so we don't need this requirement to prevent
ambiguity, and can only really check it retrospectively after name
resolution.  Emit a portability warning when appropriate.

Differential Revision: https://reviews.llvm.org/D145100