Fix rust_bindgen's clang include path and dep wiring (#4218)

rust_bindgen rebuilds clang's command line from the cc_toolchain, but
only keeps flags from an allowlist, and -idirafter isn't on it.
toolchains_llvm passes every sysroot and builtin include directory that
way (together with -nostdinc, so clang won't rediscover them on its
own), which leaves bindgen parsing headers against a truncated include
path.

Whatever survives that filter is still a C++ compile's flags being
handed to a C parse, so clang warns "argument unused during compilation"
for each one, burying the real output. Add
-Wno-unused-command-line-argument the same way the rule already adds
-Wno-unknown-warning-option; both are clang-only, which is fine since
everything after `--` goes to the clang bindgen forcibly uses.

Finally, rules_rust now warns that a CcInfo-only target in a
rust_library's 'deps' should be in 'link_deps'. Two of the targets the
macro generates trip that: the thunks cc_library, and the bindgen target
itself when merge_cc_lib_objects_into_rlib is off. Move those.

The bindgen target has to stay in 'deps' when that flag is on (the
default). There it links cc_lib into the rlib through a BuildInfo
carrying -lstatic=/-Lnative= flags, and deliberately withholds cc_lib's
libraries from its CcInfo so nothing downstream links them twice.
transform_link_deps keeps only CcInfo, so routing it through 'link_deps'
drops the objects and the binary fails to link. It also never warned,
since BuildInfo already marks it as a Rust target.

Signed-off-by: Austin Schuh <austin.linux@gmail.com>
Co-authored-by: Krasimir Georgiev <krasimir@google.com>