`clang++` defaults to `-stdlib=libc++` on OpenBSD. When building with both
`clang` and `libcxx` included, the freshly built `clang++` uses the system
version of the `libc++` headers. However, this is from the bundled `libc++`
22.1.8, thus inconsistent with the `libc++` being built.
OpenBSD has its own version of addLibCxxIncludePaths which just includes
`/usr/include/c++/v1`.
This patch removes OpenBSD::addLibCxxIncludePaths in favour of the generic
version in `Gnu.cpp`.
(cherry picked from commit 1204f6b4e9d6bef8ec319d121942dca5cbf94a26)
Brad Smith
093145f1e2