Three fixes/enhancements to the build script.

1. Take the input file from args.cfile instead of argv[1], so that
--llvmbin doesn't get added to the clang and opt command lines.

2. Iterate over the multiple files in args.cfile instead of just
processing one file.

3. Check $LLVM_BIN_PATH for clang/opt, for environments where
clang/opt are not officially installed.
Add a test that makes a call with a void return type.