refers to the "prefix" directory, i.e., one level above "bin". LLVMGCCPATH
is used as the directory containing the llvm-gcc executable, so add a "/bin"
suffix to get from LLVMGCCDIR to LLVMGCCPATH.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66823 91177308-0d34-0410-b5e6-96231b3b80d8