Regression compared to native: Using `-warnings-as-errors` & `-suppress-warnings` results in build error

[MahdiBM]

https://github.com/swift-dns/swift-endpoint/actions/runs/28789065587/job/85362846133?pr=48

[115 / 278] CNIOLinux
error: conflicting options '-warnings-as-errors' and '-suppress-warnings'
error: conflicting options '-Wwarning' and '-suppress-warnings'

the native build system works just fine on 6.3. 6.4 nightlies though result in the build error above.

This is with

.treatAllWarnings(as: .error),
.treatWarning("StrictMemorySafety", as: .warning),

This usage is explicitly allowed in the proposal:
https://github.com/swiftlang/swift-evolution/blob/b8e1751ca343d1e50a98cc4e3c06298afe6f6556/proposals/0480-swiftpm-warning-control.md?plain=1#L80

[somiljain2006]

I started looking into this in the swift-build. So far, I haven't found any code in swift-build that synthesizes SWIFT_TREAT_WARNINGS_AS_ERRORS, SWIFT_WARNINGS_AS_WARNINGS_GROUPS, or SUPPRESS_WARNINGS; the repository mostly defines these as PIF/build settings and compiler options. That makes me suspect the conflicting build settings may already be present in the PIF that swift-build receives, rather than being introduced during task construction.