[BigString] Fix accidentally quadratic BigString.init

[lorentey]

When ingesting a String instance, BigString assumes that the input string has a reasonably efficient UTF-8 view.

Unfortunately, that is very much not the case when the input happens to be backed by a bridged NSString object — it appears that in this case, the ingester loop invokes some operation(s) with linear complexity in the size of the entire input, rendering the ingester’s overall complexity quadratic.

The BigString ingester is only expected to operate within a single chunk at the time. It’s unclear precisely which operation triggers the quadratic behavior; ideally we should figure it out and resolve it with a more targeted fix.

In the meantime, a blunt stopgap fix is to force-transcode the input string to UTF-8 at the time the ingester is initialized. This unnecessarily wastes some (temporary) memory on holding the transcoded string, but it avoids the quadratic cliff.

Before:

After:

rdar://131176282

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test