[libc++] graph_header_deps.py: Update the computing of "root" (public) headers.

The "root nodes" of the graph are displayed in bold. My intent here
was to bold just the public-API headers, e.g. <vector> and
<experimental/coroutine> and <stdlib.h>, but not helper headers
such as <__functional_base> and <__iterator/next.h>. However,
the recent mass helper-header-ification has exposed defects in
this logic: all the new helpers were ending up bolded! Fix this.
Also, add <__undef_macros> to the list of headers we don't display
by default (like <__config>); it's not interesting to see those edges.

Also, add a sample `dot` command line to the `--help` text.

diff --git a/libcxx/utils/graph_header_deps.py b/libcxx/utils/graph_header_deps.py
index 1cf0015..bccd22c 100755 (executable)
--- a/libcxx/utils/graph_header_deps.py
+++ b/libcxx/utils/graph_header_deps.py
@@ -14,7 +14,7 @@ import sys
 
 
 def is_config_header(h):
-    return os.path.basename(h) in ['__config', '__libcpp_version']
+    return os.path.basename(h) in ['__config', '__libcpp_version', '__undef_macros']
 
 
 def is_experimental_header(h):
@@ -37,7 +37,7 @@ def list_all_roots_under(root):
     result = []
     for root, _, files in os.walk(root):
         for fname in files:
-            if '__support' in root:
+            if os.path.basename(root).startswith('__') or fname.startswith('__'):
                 pass
             elif ('.' in fname and not fname.endswith('.h')):
                 pass
@@ -155,11 +155,15 @@ def get_graphviz(graph, options):
 
 
 if __name__ == '__main__':
-    parser = argparse.ArgumentParser(description='Produce a dependency graph of libc++ headers, in GraphViz dot format.')
+    parser = argparse.ArgumentParser(
+        description='Produce a dependency graph of libc++ headers, in GraphViz dot format.\n' +
+                    'For example, ./graph_header_deps.py | dot -Tpng > graph.png',
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+    )
     parser.add_argument('--root', default=None, metavar='FILE', help='File or directory to be the root of the dependency graph')
     parser.add_argument('-I', dest='search_dirs', default=[], action='append', metavar='DIR', help='Path(s) to search for local includes')
     parser.add_argument('--show-transitive-edges', action='store_true', help='Show edges to headers that are transitively included anyway')
-    parser.add_argument('--show-config-headers', action='store_true', help='Show headers named __config')
+    parser.add_argument('--show-config-headers', action='store_true', help='Show universally included headers, such as __config')
     parser.add_argument('--show-experimental-headers', action='store_true', help='Show headers in the experimental/ and ext/ directories')
     parser.add_argument('--show-support-headers', action='store_true', help='Show headers in the __support/ directory')
     parser.add_argument('--show-individual-line-counts', action='store_true', help='Include an individual line count in each node')