[PyTorch] Update SmallVector from LLVM (#69110)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69110
I pasted the current LLVM code, reapplied the modifications listed in the code comments, caught a few more in the diff/build process. The trivially copyable detection is different now; if gcc builds fail, will try reverting to C10_IS_TRIVIALLY_COPYABLE or copying what LLVM is doing.
The motivation for this change is that, as noted in an existing comment, C10_IS_TRIVIALLY_COPYABLE did the wrong thing for std::unique_ptr, which caused problems with D32454856 / #68412.
ghstack-source-id: 145327773
Test Plan: CI
Reviewed By: bhosmer, mruberry
Differential Revision: D32733017
fbshipit-source-id: 9452ab90328e3fdf457aad23a26f2f6835b0bd3d
