clang-format: adjust to sort C++ headers per style(9)

Many standard C++ headers do not have the .h file extension: some, such as
iostream, lack it; others have a .hpp file extension. Moreover, some
projects (like ATF/Kyua) also contain .ipp files, which are C++ "interface"
files.

Relax the regular expression to ensure that non-traditional C "system"
headers, C++ headers, etc, with angle brackets are sorted before
"local" headers.

MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D54401