Change symlink error to warning (#3490)

Motivation: Symlinking oldBuildPath is not a vital step of swift build, but emitting an error will depress the run and test commands. This largely troubled Windows users, since symlinking requires elevated privileges on Windows.

Modifications: Codes of linking oldBuildPath is now wrapped with do-catch to capture the error and change it into a warning.

Result: If linking oldBuildPath fails, SwiftPM will emit a warning instead of an error.

Author: stevapple

Sources/Build/BuildOperation.swift

@@ -137,9 +137,18 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
             component: buildParameters.configuration.dirname
         )
         if localFileSystem.exists(oldBuildPath) {
-            try localFileSystem.removeFileTree(oldBuildPath)
+            do { try localFileSystem.removeFileTree(oldBuildPath) }
+            catch {
+                diagnostics.emit(warning: "unable to delete \(oldBuildPath), skip creating symbolic link: \(error)")
+                return
+            }
+        }
+
+        do {
+            try localFileSystem.createSymbolicLink(oldBuildPath, pointingAt: buildParameters.buildPath, relative: true)
+        } catch {
+            diagnostics.emit(warning: "unable to create symbolic link at \(oldBuildPath): \(error)")
         }
-        try localFileSystem.createSymbolicLink(oldBuildPath, pointingAt: buildParameters.buildPath, relative: true)
     }
 
     /// Compute the llbuild target name using the given subset.