LLVM 8, load dynamically from multiple shared libraries

Add API support for LLVM 8
When using dynamic loading, you can now specify multiple libraries to search through via LLVM.load, e.g.
LLVM.load("libLLVM.so", "libLTO.so"). This change was required to support dynamically loading lto/thinlto functionality, which is generally not available through the main LLVM shared library. Due to this, you can also now dynamically load when the main shared library has been split into multiple shared libraries, just list all of them. If auto loading is used or no libraries are passed toLLVM.load then llvm-d will now also try to load the LTO shared library.
Backwards incompatible change of default behavior: When dynamically loading without explicitly set filename(s), search for a main LLVM shared library without the version in its name (so e.g. libLLVM.so instead of libLLVM-8.0.0.so).

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