Designate _SendableMetatype as a marker protocol

[natecook1000]

This change addresses a source break around conditional conformances to ExpressibleByArgument and ParsableArgument that I didn't anticipate, and eliminates the unneeded runtime presence of _SendableMetatype.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test

[natecook1000]

Merging over the diagnosed API break to _SendableMetatype, which is intentional.