If the .mozconfig sets CC or CXX to clang, 'mach bootstrap' should include 'clang' in the set of packages it installs. Moreover, if the .mozconfig contains 'ac_add_options --enable-clang-plugin', 'mach bootstrap' should include 'libclang-dev' and 'libedit-dev' in the set of packages it installs, as they are dependencies of building the clang static analysis plugin.
Now that we need to generate Stylo bindings, mach bootstrap should always install clang. On x86_64 Linux, it already does (from Mozilla). On e.g. ARMv7 Linux, it doesn't. It appears that on Ubuntu 16.04, the distro package for clang-4.0 works, so it would be nice to install it among all the other distro packages that mach bootstrap installs.
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