[CMake] Revive LLVM_*_DIRS variables

This is a partial revert of r280013. Brad King pointed out these variable names are matching CMake conventions, so we should preserve them.

I've also added a direct mapping of the LLVM_*_DIR variables which we need to make projects support building in and out of tree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280380 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed