CI fails for releases/2.6

One of the runners (debian:10 clang-7-libcpp-17) fails with a segfault in clang. The latest commit on releases/2.6 passed the CI check back then so something in the setup doesn't seem to have stayed constant. Maybe a new Docker image? See !392 (closed) (which introduces a small white space change, I didn't know or didn't the right to retrigger the CI otherwise) and the corresponding job is https://gitlab.dune-project.org/core/dune-istl/-/jobs/156290 (pipeline: https://gitlab.dune-project.org/core/dune-istl/pipelines/27471 or https://gitlab.dune-project.org/core/dune-istl/pipelines/27470 for the latest releases/2.6 commit). The tests pass on all other runners which is why I cancelled them to save resources. Feel free to trigger retries.

The first error seems to be a segfault in clang

/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(_ZN4llvm3sys15PrintStackTraceERNS_11raw_ostreamE+0x1f)[0x7fe7bcb65c2f]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(_ZN4llvm3sys17RunSignalHandlersEv+0x50)[0x7fe7bcb64160]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(+0x9cef42)[0x7fe7bcb65f42]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x12730)[0x7fe7bfbbe730]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(_ZN4llvm19MachineRegisterInfo22addRegOperandToUseListEPNS_14MachineOperandE+0x28)[0x7fe7bce27a98]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(_ZN4llvm12MachineInstr10addOperandERNS_15MachineFunctionERKNS_14MachineOperandE+0x2ae)[0x7fe7bcdc8bbe]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(+0x225b6a5)[0x7fe7be3f26a5]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(+0x22590ac)[0x7fe7be3f00ac]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(+0x2258cbc)[0x7fe7be3efcbc]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(_ZN4llvm19MachineFunctionPass13runOnFunctionERNS_8FunctionE+0x89)[0x7fe7bcdc5df9]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(_ZN4llvm13FPPassManager13runOnFunctionERNS_8FunctionE+0x258)[0x7fe7bcc56668]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(_ZN4llvm13FPPassManager11runOnModuleERNS_6ModuleE+0x33)[0x7fe7bcc568c3]
/usr/lib/x86_64-linux-gnu/libLLVM-7.so.1(_ZN4llvm6legacy15PassManagerImpl3runERNS_6ModuleE+0x2eb)[0x7fe7bcc56c8b]
/usr/lib/llvm-7/bin/clang(_ZN5clang17EmitBackendOutputERNS_17DiagnosticsEngineERKNS_19HeaderSearchOptionsERKNS_14CodeGenOptionsERKNS_13TargetOptionsERKNS_11LangOptionsERKN4llvm10DataLayoutEPNSE_6ModuleENS_13BackendActionESt10unique_ptrINSE_17raw_pwrite_streamESt14default_deleteISM_EE+0x4eb5)[0x6f21e5]
/usr/lib/llvm-7/bin/clang[0xc344a5]
/usr/lib/llvm-7/bin/clang(_ZN5clang8ParseASTERNS_4SemaEbb+0x215)[0x10b8415]
/usr/lib/llvm-7/bin/clang(_ZN5clang14FrontendAction7ExecuteEv+0x3f)[0xad243f]
/usr/lib/llvm-7/bin/clang(_ZN5clang16CompilerInstance13ExecuteActionERNS_14FrontendActionE+0x4f8)[0xa93738]
/usr/lib/llvm-7/bin/clang(_ZN5clang25ExecuteCompilerInvocationEPNS_16CompilerInstanceE+0x676)[0xb608d6]
/usr/lib/llvm-7/bin/clang(_Z8cc1_mainN4llvm8ArrayRefIPKcEES2_Pv+0x5b4)[0x6aa014]
/usr/lib/llvm-7/bin/clang(main+0x2727)[0x6a84b7]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xeb)[0x7fe7bbcd909b]
/usr/lib/llvm-7/bin/clang(_start+0x2a)[0x6a5ada]

clang: error: unable to execute command: Segmentation fault (core dumped)
clang: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 7.0.1-8 (tags/RELEASE_701/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
clang: note: diagnostic msg: PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script.

but there is also others in the log which might be relevant, like:

/builds/core/dune-istl/dune/istl/eigenvalue/test/matrixinfo.hh:105:5: error: SSE2 register return with SSE2 disabled