Heads up: rewrite of CompilerRT's CMake build system

Hello folks!

I just wanted to give everyone a heads up about this. I spoke with Doug about this about a month ago and he told me to charge boldly forward, so the commits will be flying RSN.

Essentially, the compiler-rt CMake build system is largely non-functional. Major portions are just commented out, I couldn’t get it to actually build anything. Daniel has mostly been maintaining and updating the Makefile build system.

I’d like to get compiler-rt building as part of normal LLVM/Clang/Compiler-RT checkouts. Manuel and I have hacked together a prototype and everything seems to work, so I’m going to be removing the old cmake bits and adding the new ones over the next day or two. Once it’s all working and tested, I’ll remove the blacklist that prevents recursing into compiler-rt when building with LLVM & Clang, and add conditional bits to the Clang build to install the built libraries in appropriate places.

If this is going to cause anyone pain or suffering, shout out, chime in, and let me know about your use case so we can figure out the best way to support it.

-Chandler

As one important note here – I think this will remove support for building the BlocksRuntime tree in compiler-rt. Currently there is a CMake build for it, but no Makefile support.

However, using the existing cmake build, I’m unable to actually build the BlocksRuntime, on either darwin or linux, so I think this is irrelevant. Please let me know if removing this breaks anything for folks.