* LDC: Upgrade from v1.13 to v1.17
* LDC: Fix missing symbols in druntime's core.sys.posix.unistd
* LDC: Fix repeated llvm-config help output
* LDC: Revise build script
1) Use an 'installed' (ninja install) LLVM for LDC.
2) Cross-compile LLVM before invoking CMake for LDC, so that the LLVM
dir is actually populated (previously: just fake bin/llvm-config).
3) Include dlang tools ddemangle and dustmite in the final package too,
just like official LDC packages. Cmdline flags have been synced with
upstream.
* Add hello-world smoke test
* LDC: Upgrade LLVM from v7.0.1 to v8.0.1
