Add SwiftBuild coverage support

Ensure the SwiftBuild build systam has feature parity with the Native
build system as it relates to coverage.

Fixes: #9077
Issue: rdar://159461439

Author: bkhouri

Sources/Commands/SwiftTestCommand.swift

@@ -619,7 +619,6 @@ public struct SwiftTestCommand: AsyncSwiftCommand {
             }
         }
         args += ["-o", productsBuildParameters.codeCovDataFile.pathString]
-
         try await AsyncProcess.checkNonZeroExit(arguments: args)
     }
 

Sources/SwiftBuildSupport/SwiftBuildSystem.swift

@@ -925,8 +925,8 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
 
     private static func constructTestingSettingsOverrides(from parameters: BuildParameters.Testing) -> [String: String] {
         var settings: [String: String] = [:]
-        // TODO: enableCodeCoverage
-        // explicitlyEnabledTestability
+
+        settings["CLANG_COVERAGE_MAPPING"] = parameters.enableCodeCoverage ? "YES" : "NO"
 
         switch parameters.explicitlyEnabledTestability {
         case true: