[flang] Extension: initialization of LOGICAL with INTEGER & vice versa

We already accept assignments of INTEGER to LOGICAL (& vice versa)
as an extension, but not initialization.  Extend initialization
to cover those cases.

(Also fix misspelling in nearby comment as suggested by code reviewer.)

Decouple an inadvertent dependence cycle by moving two
one-line function definitions into a header file.

Differential Revision: https://reviews.llvm.org/D117159