CI: Separate devel and clang-asan builds
This build pass takes long enough that it makes sense to parallelize.
CI: Don't run scan-build
scan-build doesn't understand __attribute__((cleanup))
(see https://bugs.llvm.org/show_bug.cgi?id=3888) so it will report
increasingly many false-positive memory leaks in this codebase.
It's sufficiently time-consuming that it doesn't make sense to run it
as part of our usual pipeline.