[compiler-rt] Per-component build targets not exposed with LLVM_ENABLE_RUNTIMES

[BStott6]

When building the runtimes by passing compiler-rt to LLVM_ENABLE_PROJECTS, build targets are created for each sub-component, for example tysan or check-asan. However, this method of enabling the runtimes is deprecated, with the recommended way being to pass LLVM_ENABLE_RUNTIMES=compiler-rt. With this method, targets to build and test all of Compiler-RT are created, but the individual ones, which are really useful when working on a single sanitizer, are missing.

[BStott6]

I've spent some time reading the CMakeLists trying to work out why this is the case; it seems that the SUB_COMPONENTS global property used by llvm/runtimes/CMakeLists.txt to create the sub-component targets is empty, and that this is supposed to be populated by runtimes/CMakeLists.txt, which doesn't seem to be running.

[efriedma-quic]

This is probably complicated to implement: it's a recursive CMake invocation, and therefore the make/ninja targets are completely separate. It might be possible to add a sort of "passthrough", but I'm not sure what that would look like.

You can always just do a standalone build. (https://compiler-rt.llvm.org/ has instructions.)