[flang] Use LLVM's flags

Summary:
The only difference is that LLVM_ENABLE_WERROR is set to OFF
by default, but we enable this in a standalone flang build

This commit fixes some windows issues with the flags

Reviewers: DavidTruby, jdoerfert, sscalpone

Reviewed By: DavidTruby, sscalpone

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

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D78306