This patch allows a target to call a function add_llvm_unittest_inputs() with a list of files. This list of files will be copied from the source directory to the binary directory. Then, it adds a function to SupportHelpers that computes the input directory folder, which a unittest can then use to construct a path to the inputs folder.
LLDB has had this for a while, but now I need it in LLVM. Since we have more than one user now, having this in LLVM makes sense, and as a followup we can port LLDB's uses over to the new LLVM code.