Arch Linux

This is a follow-up to forum post https://bbs.archlinux.org/viewtopic.php?id=212643

Consider this upgrade proposal from 2016-05-12, comprised of a "llvm set" [clang, llvm, llvm-libs] and a "mesa set" [mesa, mesa-libgl, mesa-vdpau]:

extra/clang 3.7.1-1 -> 3.8.0-1
extra/llvm 3.7.1-1 -> 3.8.0-1
extra/llvm-libs 3.7.1-1 -> 3.8.0-1
---
extra/mesa 11.2.1-1 -> 11.2.2-1
extra/mesa-libgl 11.2.1-1 -> 11.2.2-1
extra/mesa-vdpau 11.2.1-1 -> 11.2.2-1

I have two problems with this upgrade:

1. Upgrading *either or both* sets causes gnome-session-binary to fail to start (landing to GDM's graphical "Oops. Something went wrong" screen).
Attached is the full journal for such a boot. Maybe of interest is the following line:
May 13 17:26:15 x gnome-session[665]: gnome-session-binary[665]: WARNING: software acceleration check failed: Child process exited with code 1

Downgrading both sets works around the issue.

2. Upgrading the llvm set causes compile-time segfaults. For example, here is how building aur/veracrypt-1.17-4 fails:
[abridged...]
Compiling Time.cpp
ar: `u' modifier ignored since `D' is the default (see `U')
ar: /usr/lib/libstdc++.so.6: version `GLIBCXX_3.4.22' not found (required by /usr/lib/libLLVM-3.8.so)
/tmp/yaourt-tmp-ronj/aur-veracrypt/src/src/Build/Include/Makefile.inc:67: recipe for target 'Platform.a' failed
make[1]: *** [Platform.a] Segmentation fauilt (core dumnped)
make[1]: *** Deleting file 'Platform.a'
Makefile:292: recipe for target 'all' failed
make: *** [all] Error 2

Here too, downgrading the llvm set fixes the issue.

Environment:
- This is an x86_64 Dell XPS1645 laptop with an AMD Mobility Radeon HD 5730 / 6570M gpu
- uname -a = Linux x 4.5.4-1-ARCH #1 SMP PREEMPT Wed May 11 22:21:28 CEST 2016 x86_64 GNU/Linux
- System is fully upgraded as of 2016-05-16 (save for the offending packages of course)
- I didn't test whether this is an upstream or distro bug.

Potentially-related recent forum posts:
- Mesa 11.2.2 breaks Intel graphics, https://bbs.archlinux.org/viewtopic.php?id=212566
- gdm hangs in loop, https://bbs.archlinux.org/viewtopic.php?id=212219
- GDM fails to appear, https://bbs.archlinux.org/viewtopic.php?id=212270
- Mesa 11.2.2 breaks MATLAB graphics, https://bbs.archlinux.org/viewtopic.php?id=212532

Feel free to ask for more information.