[flang][msvc] Do not use gcc/clang command line options for msvc.

The command line options `-Wno-error` and `-Wno-unused-parameter` are specific to gcc/clang, do not use them when compiling with other compilers.

This patch is part of the series to [[ http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html | make flang compilable with MS Visual Studio ]].

Reviewed By: isuruf

Differential Revision: https://reviews.llvm.org/D85355