Work around a llvm-config bug by creating a versioned symlink to libLLVM

[maleadt]

Attempted fix for #29981

[maleadt]

FWIW, I've seen this recommended by some other projects, eg. https://github.com/AccelerateHS/accelerate-llvm/blob/master/README.md#building-from-source

[maleadt]

Turns out CI is pretty worthless here, since the macOS builder uses BinaryBuilder.
Confirmed to work locally though, with llvm-config now reporting --shared-mode as shared (and as a result listing libLLVM.dylib as --libfiles).
More importantly, the julia binary is back to 3MB, and LLVM.jl works again.

Is this good to go? Also confirmed to work with binary-dist:

$ tar -tvf julia-96ce5ba221-mac64.tar.gz
lrwxr-xr-x julia/staff       0 2018-11-14 11:38 julia-96ce5ba221/lib/julia/libLLVM-6.0.dylib -> libLLVM.dylib
-rwxr-xr-x julia/staff 42447980 2018-11-14 11:32 julia-96ce5ba221/lib/julia/libLLVM.dylib
-rwxr-xr-x julia/staff   3213496 2018-11-14 11:46 julia-96ce5ba221/lib/libjulia.1.1.dylib

[vchuravy]

I kicked off https://build.julialang.org/#/buildrequests/5154?redirect_to_build=true to check if it works on the buildbots as well.

[maleadt]

I kicked off https://build.julialang.org/#/buildrequests/5154?redirect_to_build=true to check if it works on the buildbots as well.

Great, thanks! Looks like that went through and contains the symlink:

a julia-98b0264fb8/lib/julia/libLLVM-6.0.dylib
a julia-98b0264fb8/lib/julia/libLLVM.dylib

There's some test failures but those seem unrelated:

Error in testset FileWatching:
Test Failed at /Users/osx/buildbot-tabularasa/worker/tester_osx64/build/share/julia/stdlib/v1.1/FileWatching/test/runtests.jl:332
  Expression: events.changed && (!(events.timedout) && !(events.renamed))
Error in testset FileWatching:
Test Failed at /Users/osx/buildbot-tabularasa/worker/tester_osx64/build/share/julia/stdlib/v1.1/FileWatching/test/runtests.jl:332
  Expression: events.changed && (!(events.timedout) && !(events.renamed))