Fix race condition in clang_macro_fallback

[HKalbasi]

Previously, bindgen by default used the same directory of the headers as the directory for temporary files of the clang_macro_fallback feature. Aside from its other problems like making temporary noises in git or failing to remove these files on failing or aborted builds, this caused a race condition in build of retina that I spend two DAYS to figure it out.

The problem is, if you run bindgen on the same set of files multiple times simultaneously (which happened in retina since a crate was included two times in the crate graph due different features, and it ran bindgen in its build script) one of them can remove clang_macro_fallback files of the other one and break it.

I fixed it by moving the files generated by clang_macro_fallback in a new temp directory each time bindgen is invoked.

[ojeda]

Cc @jbaublitz since he is probably interested.

[jbaublitz]

@HKalbasi You may want to base this PR on #3072 as that is the PR I put up to address some problems that @ojeda bumped into while using the clang fallback code.

[HKalbasi]

I will do the rebase after your PR is merged so I don't have to rebase it multiple times in case your PR needs further changes, and to keep the diff clean if you or anyone else want to review it in the meantime.

[jbaublitz]

@HKalbasi Okay, it looks like the PR got merged.