De-duplicate references to share/llvm/cmake path

Use a LLVM_INSTALL_PACKAGE_DIR variable to hold the path and reference
it where necessary.

Contributed by Brad King.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201046 91177308-0d34-0410-b5e6-96231b3b80d8