[flang] Fix build problems on osx

Summary:
Fix build problems encountered on osx in two files.
The one in character.cpp fixes a legitimate bug that
elicited a valid warning.

Reviewers: tskeith, PeteSteinfeld, sscalpone, jdoerfert, DavidTruby

Reviewed By: tskeith, PeteSteinfeld

Subscribers: llvm-commits, flang-commits

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D82107