Fixed typo in CMakeLists.txt.

We were marking both LLVMBUILDOUTPUT and LLVMBUILDERRORS as
ERROR_VARIABLES when clearly LLVMBUILDOUTPUT should be marked as
OUTPUT_VARIABLE.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188444 91177308-0d34-0410-b5e6-96231b3b80d8