Try llvm-config binaries with version suffix.

This allows us to pick up the newest available version on
several common Debian derivatives.

[Cherry-picked 2b4f49b4e from master.]
Search LLVM_ROOT_DIR before system directories.
Fold in MinGW Phobos test update.
Merge branch 'release-0.11.0'.

Conflicts:
	cmake/Modules/FindLLVM.cmake
	runtime/phobos