bindgen: Pass `-resource-dir` Clang flag to bindgen-cli. (#3591)

For some cc_toolchain configurations, this is required to find the
builtin
headers.

So far, the bindgen rules had assumed that the builtin headers can be
found through `cc_toolchain.built_in_include_directories`. This is,
however, not guaranteed to be the case; indeed,
`built_in_include_directories` is not really intended for finding
builtin headers but merely for filtering out diagnostics in builtin
headers.

Clang itself finds the resource directory relative to its own binary,
and then finds the builtin headers in the "include" subdirectory of the
resource directory. bindgen-cli may, however, be located in a different
directory than Clang, so we pass a `-resource-dir` flag on the command
line.