third_party/BUILD.gn was adding compile flags to suppress failures that
were GCC/Clang specific and caused MSVC to error out. Replace them by
suppression using MSVC's flags.
Change-Id: Ia25ae315dcf8904dbfd8eff877065a24e8c88769
This includes a bunch of fixes for clang warnings in Windows specific
code that was only compiled by MSVC previously. This also tidies up some
BUILD.gn issues on Windows.
This required adding some missing dependencies, splitting public headers
of libdawn_[native|wire] so they aren't hidden in the
libdawn_[native|wire]_sources targets, and making unittests depend on
sources directly instead of static libraries (which is almost equivalent).
As a byproduct, Empty.cpp is no longer needed and is removed.
Corentin Wallez