GHA: setup swift toolchain version information

[compnerd]

This embeds a compiler version tag into the output so that we can easily identify the toolchain.

[Copilot]

Pull Request Overview

This PR embeds the Swift toolchain version and tag into the build output to improve traceability.

• Adds a new SWIFT_TOOLCHAIN_VERSION compile-time definition combining swift_version and swift_tag
• Ensures the CI workflow captures both version and tag metadata
• Updates the GitHub Actions workflow file to include the new input in the build definitions

Comments suppressed due to low confidence (2)

.github/workflows/swift-toolchain.yml:1178

• [nitpick] Using a space to concatenate swift_version and swift_tag can make downstream parsing brittle. Consider joining them with a consistent delimiter (e.g., -) or templating them in a single string in the workflow inputs to avoid ambiguous splits.

                -D SWIFT_TOOLCHAIN_VERSION="${{ inputs.swift_version}} ${{ inputs.swift_tag }}" `

.github/workflows/swift-toolchain.yml:1178

• Verify that the swift_tag input is declared in the workflow_dispatch inputs. If it isn't defined, this reference will be unresolved and cause the build to fail.

                -D SWIFT_TOOLCHAIN_VERSION="${{ inputs.swift_version}} ${{ inputs.swift_tag }}" `