[llvm] [Runtimes] Pass through per-runtime CMake options for target runtimes (PR #194105)

[Petr Hosek via llvm-commits]

petrhosek wrote:

`PASSTHROUGH_PREFIXES` specifies prefixes of variables that should be passed to the subbuild. After this change we would pass all runtime prefixes to all per-target runtime builds by default which can have undesirable consequence, especially on platforms like Darwin. For example, in our case all of these variables are now passed to all per-target runtime builds which is wrong:
https://github.com/llvm/llvm-project/blob/504a11278112afbedb890c512fb9751a40bf5210/clang/cmake/caches/Fuchsia-stage2.cmake#L70-L85

Can you please revert this change and explain what's the issue you're trying to address so we can advise on a correct solution?

https://github.com/llvm/llvm-project/pull/194105