Improve unsupported lit test config for print-changed=dot-cfg

[jamieschmeiser]

Summary

Expand testing of necessary features for print-changed=dot-cfg.

Expand the testing for whether the lit tests for print-changed=dot-cfg
are supported to include checking whether dot supports pdf output.

[jamieschmeiser]

ping

[hvdijk]

Apologies, forgot about this, and thanks. I personally think it might have been slightly simpler to check whether dot -Tpdf </dev/null (or an equivalent that avoids the shell) exits successfully, instead of having to parse dot's output, but since you already have it working this way, I'm also okay if you want to leave it as it is.

[jamieschmeiser]

The dot documentation I saw recommended this method of checking whether an output format was supported.

llvm/test/Other/ChangePrinters/DotCfg/lit.local.cfg

 import os
+import subprocess
 
+def have_needed_dot_support():
   if not os.path.exists('/usr/bin/dot'):
+    return False
+
+  try:
+    ld_cmd = subprocess.Popen(['/usr/bin/dot', '-T?'],
+                              stdout=subprocess.PIPE,
+                              stderr=subprocess.PIPE)
+    ld_err = ld_cmd.stderr.read().decode()
+    ld_cmd.wait()
+  except:
+    return False
+
+  return 'pdf' in ld_err
+
+if not have_needed_dot_support():
   config.unsupported = True