Revert workaround landed in #1139 (re-enabling SymbolLinkageMarkers) once unblocked

[stmontgomery]

Description

Tracks reverting the workaround applied to this project in #1139. In particular, this means re-enabling the experimental Swift feature SymbolLinkageMarkers.

The problem was affecting macOS builds of the swift-testing package.

Additional information

Example log showing the failure this caused before the workaround was applied:

https://ci.swift.org/job/pr-swift-testing-macos/1914/consoleFull

[stmontgomery]

Blocked by rdar://154332540

[stmontgomery]

This should be unblocked once swiftlang/swift#82561 lands in the relevant toolchains