ci: Add scan-build job in a new ‘analysis’ pipeline stage
ci: Add scan-build job in a new ‘analysis’ pipeline stage

Use Meson’s support for running scan-build (Clang’s static analyser)
against the build, so that we get static analysis of each pipeline. Add
it in a new pipeline stage, after code coverage, so that we don’t waste
resources on it unless the unit tests pass — a static analysis build
takes perhaps 10× as long as a normal GCC build.

https://mesonbuild.com/howtox.html#use-clang-static-analyzer

Currently, the static analysis results are uploaded as artifacts, but
the job will always succeed (regardless of whether there are any bugs
found in the analysis).

Currently, a large number of reports are outputted by the analyser,
which need to be fixed before we can gate the pipeline on it.
Furthermore, in order to get scan-build to exit with a non-zero status
if any bugs are found, we need to depend on Meson ≥ 0.49.0, which
contains the fix https://github.com/mesonbuild/meson/issues/4334,
allowing us to add the following to .gitlab-ci.yml:
```
variables:
  # Exit with a non-zero status if any bugs are found
  SCANBUILD: "scan-build --status-bugs"
```

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib