Ensure that LLVM-created .a files don't contain absolute paths
Ensure that LLVM-created .a files don't contain absolute paths

llvm::writeArchive() will happily create .a files with absolute paths if you pass it files specified that way; some versions of 'ar' can't reliably deal with these (e.g. 'ar x' on OS X will refuse to extract these files in most cases). Work around this to ensure that we always produce canonically useful archives.

Former-commit-id: 70fde01b340ce3dcd39a93bc81fe39d776cef176
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide