remote-exec: Only copy command line arguments which name files that exist.

Speculative fix for failing buildbot:
https://lab.llvm.org/buildbot/#/builders/193/builds/8961

Author: pcc

llvm/utils/remote-exec.py

@@ -130,11 +130,12 @@ def main():
                 ]
             )
         else:
-            # Copy only the files, which are specified in the command line.
-            # Copy them to remote host one by one.
+            # Copy only files which are specified in the command line and exist on
+            # the host. Copy them to the remote host one by one.
             for x in commandLine:
-                _, f = os.path.split(x)
-                subprocess.check_call(scp(args, x, pathOnRemote(f)))
+                if os.path.exists(x):
+                    _, f = os.path.split(x)
+                    subprocess.check_call(scp(args, x, pathOnRemote(f)))
 
         # Make sure all executables in the remote command line have 'execute'
         # permissions on the remote host. The host that compiled the test-executable