Support targeting a specific llvm branch

[jakesays-old]

To overcome the recent llvm version bump the tools build should be able to target a specific llvm/clang/libs branch.

[jwinarske]

The problem that needs solving here, is setting the compiler-rt CMAKE_INSTALL_PREFIX to the proper Clang version.

I experimented with doing this in the toolchain file, only setting CMAKE_INSTALL_PREFIX is ignored in the toolchain file. It needs to get passed in.

My conclusion earlier today, is that this should really be handled in the compiler-rt project. It has the version info available (I think). I'm adding this to my LLVM upstream agenda.

I'm open to an interim workaround, if you have something in mind.

[jakesays-old]

I just added to my PR support for a specific llvm branch, which should handle the install prefix issue (i think).

If you set -DUSE_LLVM_BRANCH=release_80 it will check out that branch. it defaults to trunk.

After thinking about this more it doesn't solve the issue you're trying to solve. I want to be able to pin the toolchain to a specific version instead of dealing with the unstable trunk. I can't rely on trunk for doing real work.

[jakesays-old]

Also, after clang is built you can parse the version from clang --version if you need it for compiler-rt.

EDIT: hmm. disregard. it's available from llvm-config.