Only check isTargetSuitableForPlatformForIndex for workspace description

For the package and target build description we can end up incorrectly
dropping build tool dependencies since not all clients pass in the
correct dependency information. It's not clear that we actually gain
much from doing this check for the target and package build
descriptions though, its primary purpose is to avoid configuring
unsupported targets in the workspace build description where we try
to configure for all available platforms. As such, switch to only
checking for the workspace build description. Note we don't encounter
this issue in the workspace case since there we override the build
parameters for host build tools to always build for the host platform.

rdar://152012769

Author: hamishknight

Sources/SWBCore/DependencyResolution.swift

@@ -623,23 +623,15 @@ extension SpecializationParameters {
 
     /// Determines whether a target should be configured for the given platform in the index arena.
     ///
-    /// The arena is used for two purposes:
-    ///   1. To retrieve settings for a given target
-    ///   2. To produce products of source dependencies for compilation purposes (it does not produce binaries)
+    /// When building a workspace build description, we configure for all possible platforms. As such,
+    /// we want to avoid unnecessarily configuring targets for unsupported platforms. When building a
+    /// target or package description, we are only configuring for a single platform and can therefore
+    /// avoid this check since we assume any dependency will necessary.
     ///
-    /// Thus, in general if a target doesn't support a platform, we don't want to configure it for that platform. If a
-    /// dependency is not supported for the platform of the dependent, presumably the dependent will not be able
-    /// to use its products for compilation purposes, since the source products will be put in a different platform
-    /// directory and/or they will not be usable by the dependent (e.g. the module will not be importable from a
-    /// different platform). If the dependency was intended to be usable from that platform for compilation purposes,
-    /// it would be a supported platform.
-    ///
-    /// There's an exception for this for a dependent host tool, which are required for compilation and must therefore
-    /// be configured (and registered as a dependency) regardless.
+    /// Note there's an exception for this for host build tools, which are required for compilation
+    /// and must therefore be configured (and registered as a dependency) regardless
     nonisolated func isTargetSuitableForPlatformForIndex(_ target: Target, parameters: BuildParameters, imposedParameters: SpecializationParameters?, dependencies: OrderedSet<ConfiguredTarget>? = nil) -> Bool {
-        if !buildRequest.enableIndexBuildArena {
-            return true
-        }
+        guard buildRequest.buildsIndexWorkspaceDescription else { return true }
 
         // Host tools case, always supported we'll override the parameters with that of the host regardless.
         if target.isHostBuildTool || dependencies?.contains(where: { $0.target.isHostBuildTool }) == true {

Tests/SWBCoreTests/IndexTargetDependencyResolverTests.swift

@@ -949,8 +949,12 @@ import SWBUtil
             #expect(results.targets(packageProduct).map{ results.targetNameAndPlatform($0) } == ["PackageLibProduct-iphoneos"])
             #expect(results.targets(unreferencedPackageLib).map{ results.targetNameAndPlatform($0) } == ["UnreferencedPackageLib-iphoneos"])
 
-            try results.checkDependencies(of: .init(packageLib2, "iphoneos"), are: [])
-
+            switch results.graphType {
+            case .dependency:
+                try results.checkDependencies(of: packageLib2, are: [.init(packageTool, "macos")])
+            case .linkage:
+                try results.checkDependencies(of: packageLib2, are: [])
+            }
             results.delegate.checkNoDiagnostics()
         }
     }