Revert "[cmake] Add helper for finding potentially external projects"

This reverts commit r244633.

We aren't going to be able to use it because the compiler-rt build can
be built standalone without an LLVM source dir *or* an installed copy of
LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244648 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed