[flang][NFC] Address warnings from Windows build

An LLVM Flang build bot for Windows recently failed with a
bunch of warning messages.  None were from recent changes to
the Fortran compiler; I suspect that a newer (or maybe older)
version of MSVC was being used, or perhaps a different set of
compiler options were temporarily applied to the build, since
the buildbot status went back to green shortly thereafter.

Most of the warnings looked bogus to me, but some are legitimate
concerns and we might as well clean them up.  This patch does so.

Differential Revision: https://reviews.llvm.org/D111677