Fix unintentionally-quadratic COW copies on insertion

[harlanhaskins]

This fixes an accidentally-quadratic COW copy of a subgraph whenever an item is inserted.

Motivation:

For large Swift Testing test suites with many parameterized test cases, we accidentally tripped this specific pathological failure mode of the graph insertion code; when we insert a new child, we mutate each graph entry along the ancestor chain. If we mutate our local copy while leaving the existing copy in the tree, it causes a COW copy of the whole tree on every insertion.

For the attached stress test, without the .take() it takes 2 seconds on my MacBook Pro, and with the .take() it takes 40 milliseconds.

Modifications:

The bulk of this fix is to .take() the child from the dictionary (Optional.take() sets the receiver to nil and returns the previous wrapped value).

This is a very similar problem and fix to this PR in swift-driver: swiftlang/swift-driver#654

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

Nice. Please give me a chance to review on Monday before merging!

[jerryjrchen]