opencilk/v2.1-rc1 bugfixes

[neboat]

This PR fixes several bugs found in v2.1 release candidate 1, including:

• Fix compiler crash when handling of spawns of return statements. (OpenCilk/opencilk-project#194)
• Fix compiler crash lowering of task.frameaddress intrinsic when no Tapir backend is specified. (OpenCilk/opencilk-project#198)
• Fix bad ASan behavior when Cilksan/CSI and ASan are used together on some programs with stack allocations. (OpenCilk/opencilk-project#197)
• Replace CSI API for handling sync regions to propagate sync-region numbers, as the Cilksan instrumentation does.
• Ensure that Tapir lowering assigns calls to outlined helper functions with proper debug information.