Fix duplicated swiftc flags when using custom sdks

- Fixes a bug which caused extraCLIFlags from an SDK's toolset.json
  files to be repeated multiple times in child process invocations. The
  repeated flags can cause issues with some downstream tools, such as
  repeated -segaddr flags to ld64 for the same segment.

Author: rauhul

Sources/Build/BuildDescription/ClangTargetBuildDescription.swift

@@ -236,11 +236,13 @@ public final class ClangTargetBuildDescription {
         }
 
         args += buildParameters.toolchain.extraFlags.cCompilerFlags
-        // User arguments (from -Xcc and -Xcxx below) should follow generated arguments to allow user overrides
+        // User arguments (from -Xcc) should follow generated arguments to allow user overrides
         args += buildParameters.flags.cCompilerFlags
 
         // Add extra C++ flags if this target contains C++ files.
         if clangTarget.isCXX {
+            args += buildParameters.toolchain.extraFlags.cxxCompilerFlags
+            // User arguments (from -Xcxx) should follow generated arguments to allow user overrides
             args += self.buildParameters.flags.cxxCompilerFlags
         }
         return args

Sources/CoreCommands/SwiftTool.swift

@@ -680,15 +680,13 @@ public final class SwiftTool {
             let dataPath = self.scratchDirectory.appending(
                 component: destinationTriple.platformBuildPathComponent(buildSystem: options.build.buildSystem)
             )
-            var buildFlags = options.build.buildFlags
-            buildFlags.append(destinationToolchain.extraFlags)
 
             return try BuildParameters(
                 dataPath: dataPath,
                 configuration: options.build.configuration,
                 toolchain: destinationToolchain,
                 destinationTriple: destinationTriple,
-                flags: buildFlags,
+                flags: options.build.buildFlags,
                 pkgConfigDirectories: options.locations.pkgConfigDirectories,
                 architectures: options.build.architectures,
                 workers: options.build.jobs ?? UInt32(ProcessInfo.processInfo.activeProcessorCount),