[flang] Turn off FLANG_ENABLE_WERROR by default

Summary: This is a follow up to https://reviews.llvm.org/D78306

Reviewers: DavidTruby, sscalpone, jdoerfert

Reviewed By: DavidTruby

Subscribers: tskeith, Meinersbur, ChinouneMehdi, richard.barton.arm, mehdi_amini, mgorny, llvm-commits

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D81695