[flang] Finer control over warnings

Establish a set of optional usage warnings, and enable some
only in "-pedantic" mode that, in our subjective experience
with application codes, seem to issue frequently without
indicating usage that really needs to be corrected.  By default,
with this patch the compiler should appear to be somewhat less
persnickety but not less informative.

Differential Revision: https://reviews.llvm.org/D150710