Restore functionality of --sysroot on FreeBSD after b18cb9c47
After b18cb9c47, clang would sometimes prefer the host C++ includes
(e.g. in /usr/include/c++/v1) before those specified via --sysroot.
While this behavior may be desirable on Linux, it is not so on
FreeBSD, where we make extensive use of --sysroot during the build of
the base system. In that case, clang must *not* search outside the
sysroot, except for its own internal headers.
Add an override addLibCxxIncludePaths() to restore the old behavior,
which is to simply append /usr/include/c++/v1 to the specified
sysroot. While here, apply clang-format to the FreeBSD specific
toolchain files.
Fixes PR44923.
46bc58a0ff