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

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.

Compare the targets produced when compiler-rt is passed through LLVM_ENABLE_PROJECTS with those produced through LLVM_ENABLE_RUNTIMES. Identify where the per-component build and test targets are defined, then verify that individual sanitizer targets are exposed through the recommended configuration.

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