Work around temporary issue with current 6.3 snapshots

[lorentey]

OutputSpan.withUnsafeMutableBufferPointer is temporarily broken in (some of) the 6.3 snapshots that we have. Add a horrible, terrible, very bad workaround until the fix spreads to all toolchain builds.

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.

[glessard]

With apologies.

[glessard]

Why did Fatal error: UnsafeMutableRawBufferPointer has a nil start and nonzero count happen to the Windows build?

[lorentey]

I'm looking into it; the UMBP initializer is trapping. Hopefully it isn't a similar issue.

UnsafeMutableRawBufferPointer has a nil start and nonzero count

[lorentey]

(It reproduces with a recent 6.3 build on macOS, too.)

[lorentey]

Huh; OutputSpan.span returns an empty span with nil start, even though the output span has a start address.