rdmd: Fix rebuilding a still-running program on Windows

Windows locks the executable files of running processes.
While the program is running, we can't overwrite or delete
its executable file, but we can rename it out of the way.

rdmd.d

@@ -399,10 +399,11 @@ private int rebuild(string root, string fullExe,
         string workDir, string objDir, in string[string] myDeps,
         string[] compilerFlags, bool addStubMain)
 {
+    auto fullExeTemp = fullExe ~ ".tmp";
     string[] buildTodo()
     {
         auto todo = compilerFlags
-            ~ [ "-of"~fullExe ]
+            ~ [ "-of"~fullExeTemp ]
             ~ [ "-od"~objDir ]
             ~ [ "-I"~dirName(root) ]
             ~ [ root ];
@@ -455,6 +456,20 @@ private int rebuild(string root, string fullExe,
             // directory. One will fail.
             collectException(rmdirRecurse(objDir));
         }
+        yap("mv ", fullExeTemp, " ", fullExe);
+        try
+            rename(fullExeTemp, fullExe);
+        catch (FileException e)
+        {
+            // This can occur on Windows if the executable is locked.
+            // Although we can't delete the file, we can still rename it.
+            auto oldExe = "%s.%s-%s".format(fullExe, Clock.currTime.stdTime,
+                thisProcessID);
+            yap("mv ", fullExe, " ", oldExe);
+            rename(fullExe, oldExe);
+            yap("mv ", fullExeTemp, " ", fullExe);
+            rename(fullExeTemp, fullExe);
+        }
     }
     return 0;
 }