[msbuild] In 'GenerateDebugSymbols', verify that the folder we're touching into exists (#5706)

- Previously, for `Device|Debug` when passing `--dsym=false` to the mtouch extra arguments you would get this [MSB3375 failure](https://gist.github.com/VincentDondain/dce853be2acbce16011adf43a200eb09#file-gistfile1-txt-L5600).
- We are now checking that the folder we're trying to touch into exists, preventing the build failure.

msbuild/Xamarin.iOS.Tasks.Core/Xamarin.iOS.Common.targets

@@ -1006,7 +1006,7 @@ Copyright (C) 2013-2016 Xamarin. All rights reserved.
 		<!-- touch the dSYM Info.plist so that its mtime is newer than the stripped $(_NativeExecutable) -->
 		<Touch
 			SessionId="$(BuildSessionId)"
-			Condition="'$(IsMacEnabled)' == 'true' And '$(MtouchNoSymbolStrip)' == 'false' And '$(IsAppExtension)' == 'false'"
+			Condition="'$(IsMacEnabled)' == 'true' And '$(MtouchNoSymbolStrip)' == 'false' And '$(IsAppExtension)' == 'false' And Exists('$(AppBundleDir).dSYM')"
 			Files="$(AppBundleDir).dSYM\Contents\Info.plist"
 		>
 		</Touch>