Auto merge of #65459 - ecstatic-morse:graphviz-subgraph, r=estebank

Fix `-Zunpretty=mir-cfg` to render multiple items

`-Zunpretty=mir-cfg` outputs DOT to stdout for all items being compiled. However, it puts all of these items in separate `digraph`s, which means the result of redirecting that output to a file is not valid. Most dot renderers (I have tried `dot` and `xdot`) cannot render the output.

This PR checks to see if `write_mir_graphviz` will  process multiple items, and writes them each as a `subgraph` in a single, top-level `digraph`. As a result, DOT can be viewed without manually editing the output file. The output is unchanged when printing a single item (e.g.`-Zunpretty=mir-cfg=item_name`).

Here's the output of `xdot` for a rust file containing three items:
![three-items](https://user-images.githubusercontent.com/29463364/66889712-4bf62200-ef98-11e9-83b5-60faa2a300dd.png)

The borders are a result of the nonstandard–but well-supported–[`cluster` prefix](https://graphviz.gitlab.io/_pages/doc/info/lang.html) (search for "Subgraphs and Clusters"). They will not appear if your renderer does not support this extension, but the graph will still render properly.

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