The last GPLv2 GCC is 4.2.1. OpenBSD has 4.9.3 (they also have 4.2.1 and 3.3.6). Why not 5.3? Why was clang updated post 5.9? How long do they take to stabilize a release?
GCC 4.9.X [1] was the stable branch first released on 2014-04-22 with latest patch 4.9.4 released on 2015-06-26.
GCC 5.X is the next stable branch first released exactly a year after 4.9 on 2015-04-22 with latest patch 5.3 released on 2015-12-04.
LLVM 3.5 [2] was released on 2014-09-03, latest patch 3.5.2 was released on 2015-04-02.
LLVM 3.6 was released on 2015-02-27, latest patch 3.6.2 was released on 2015-07-16.
LLVM 3.7 was released on 2015-09-01, latest patch 3.7.1 was released on 2016-06-05.
6 months before the 2016-03-29 release of OpenBSD 5.9, i.e. at the end of September of 2015, GCC 5.2 was the stable version, not GCC 4.9.3, but they chose GCC 4.9.3. LLVM 3.7 was the stable release, but they chose 3.5 (I hope it's 3.5.2 though it doesn't say).
So, OpenBSD's compilers are the stable branch from ~14 months before the release, with relevant updates applied. Not Debian, but not Arch either.
1 - https://gcc.gnu.org/releases.html 2 - http://llvm.org/releases/
For example the binaries for Ubuntu 14.04 don't work if the newer GCC is already installed.
Swift compiler will fail to find the modules due to an ABI error as it will interpret the function call results from libstd++ as failure.
The workaround is to re-compile everything with the new version.