clang-format now reorders includes. It used to simply sort within
blocks, but later it added a "regroup" option. The regroup option is a
bit aggressive and does not take into account our project headers being
referenced in <system/header.h> style. (It also won't be able to
recognize the header corresponding to the source file, but perhaps we
should drop that rule.)
For now, just revert it to Preserve.
Change-Id: Ief82b5c3f91c16a8def14f91ef6bf6cde502bb79
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/42265
Reviewed-by: Adam Langley <agl@google.com>