lld 12.0.1 (redux)

[h-vetinari]

Closes #26
Closes #25

Following @isuruf's suggestion.

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[h-vetinari]

Haven't had time to figure out what's happening with all these CMake paths, and why the relevant include-directory is not being picked up despite the call being there in lld/CMakeLists.txt.

[h-vetinari]

@isuruf @xhochy
Haven't ever built LLVM before, so I'm a bit stuck here once the trivial adaptations don't work.

I find the include_directories(${LLVM_MAIN_SRC_DIR}/../libunwind/include) really weird, because it seems to be going out one folder level too far, and indeed, the windows build shows why this doesn't work:
-ID:\bld\llvm-package_1625847118277\work\..\libunwind\include

I'm have an exploratory patch for this now, but not sure if that's the right approach. In the same vein, I'm not sure what the purpose of the previous patch was - pointers appreciated.

[h-vetinari]

@isuruf, this should be ready for review. Last run was green, I just rebased to clean up my derps a bit and remove the overzealous logging again.

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