Bundle libclang with bindgen

Goal: make it super easy for people to use bindgen (and crates to depend on bindgen without inflicting downstream pain), no installing libclang manually, no configuring LIBCLANG_PATH, no making sure your libclang is the right version.

Previously, we have speculated about getting rustup to distribute libclang for us. I talked with @alexcrichton and he pointed out that we don't even need to do that to get what we want. We can bundle libclang.{so,dll,dylib} into our crate directly and then have build.rs setup LIBCLANG_PATH as needed.

To be clear, this functionality would be behind a new feature, on by default. One could always turn this feature off to get the current behavior, and for targets for which we don't have a bundled libclang available, we will also have the current behavior.

There are still some open questions:

• Should we put the bundled libclang into this repository? A different bindgen-libclangs repository?

• Should we use git lfs?

• Should the libclangs be literally bundled in the crate, and downloaded with the rest of the crate's source, or should we fetch only the target's libclang inside build.rs?

• What targets should we bundle libclang for? Tier-1 platforms? Start with one or two and then add more as we go?

Start by reading the existing build.rs flow and how bindgen currently locates libclang. Resolve the repository, packaging, download, and target-support questions listed in the issue, then define the default feature, opt-out behavior, and fallback for unsupported targets.