Fix clean.cmd with no args (dotnet#25158)

Also just copy over coreclr's clean.cmd to make it identical.

clean.cmd

@@ -2,15 +2,17 @@
 setlocal EnableDelayedExpansion
 
 set NO_DASHES_ARG=%1
+if not defined NO_DASHES_ARG goto no_help
 if /I [%NO_DASHES_ARG:-=%] == [?] goto Usage
 if /I [%NO_DASHES_ARG:-=%] == [h] goto Usage
 
+:no_help
 :: Check if VBCSCompiler.exe is running
 tasklist /fi "imagename eq VBCSCompiler.exe" |find ":" > nul
 :: Compiler is running if errorlevel == 1
 if errorlevel 1 (
-  echo Stop VBCSCompiler.exe execution.
-  for /f "tokens=2 delims=," %%F in ('tasklist /nh /fi "imagename eq VBCSCompiler.exe" /fo csv') do taskkill /f /PID %%~F
+	echo Stop VBCSCompiler.exe execution.
+	for /f "tokens=2 delims=," %%F in ('tasklist /nh /fi "imagename eq VBCSCompiler.exe" /fo csv') do taskkill /f /PID %%~F
 )
 
 :: Strip all dashes off the argument and use invariant
@@ -30,14 +32,13 @@ exit /b %ERRORLEVEL%
 
 :Usage
 echo.
+echo Usage: clean [-b] [-p] [-c] [-all]
 echo Repository cleaning script.
-echo.
 echo Options:
-echo     -b     - Deletes the binary output directory.
-echo     -p     - Deletes the repo-local nuget package directory.
-echo     -c     - Deletes the user-local nuget package cache.
-echo     -all   - Combines all of the above.
+echo     -b     - Delete the binary output directory.
+echo     -p     - Delete the repo-local NuGet package directory.
+echo     -c     - Deletes the user-local NuGet package cache.
+echo     -all   - Cleans repository and restores it to pristine state.
 echo.
-echo If no option is specified then clean.cmd -b is implied.
-
-exit /b 1
+echo ^If no option is specified then "clean -b" is implied.
+exit /b