Silence clang 3.8 warning about missing braces.
This adjusts the clang 3.8 toolchains to include -Wno-missing-braces in their flags. Clang includes -Wmissing-braces in -Wall. This has the annoying effect of requiring two pairs of braces when aggregate-initializing e.g. an std::array, see https://gitlab.dune-project.org/core/dune-common/-/jobs/79786 for an example. Gcc did removed -Wmissing-braces from -Wall precisely for this reason a long time ago. Clang 6 went a different route and refined the warning to not trigger in cases such as std::array, see https://bugs.llvm.org/show_bug.cgi?id=21629.
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