Implement Salvatore Previti's host compiler caching workaround

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// <summary>
    /// Author: Salvatore Previti - 08-April-2009.
    /// 
    /// Visual studio host compiler keeps a cache of files during the MSBuild step, so they are not readed from disk.
    /// This is ok for performance and intellisense, right, but this introduces also some subtle bug!
    /// 
    /// See forum thread "Generated Code Not Building - VS Caching Problem?" at http://social.technet.microsoft.com/Forums/en-US/msbuild/thread/14bb304d-d8e6-4227-bc2b-e2e6d4f979be?ppud=4 for more informations.
    /// This task contains a simple but effective work around to this caching bug :)
    /// 
    /// How it works:
    /// 
    /// Visual studio keeps MSBuild Tasks dll loaded also between different compilations in the same appdomain where MSBuild is executed.
    /// Due to this fact, we can keep some states (static variables) between different compilations during the same development session.
    /// This class, infact, uses a static synclocked dictionary to keep states between different compilations.
    /// Each time static function UpdateFileUsageCount is called with the same prefix and with the same filename (case insensitive), the resulting value is incremented.
    /// 
    /// There is another "feature" (bug?) in visual studio cache mechanism... &lt;Compile Include="c:\test.cs"&gt and &lt;Compile Include="c:\test.cs "&gt
    /// are considered different files, so, they are cached as two different files (look the space character in "test.cs " in the second Compile tag).
    /// MSBuild and all filesystems, instead, treat both files as the same :)
    /// 
    /// So the solution! Each time we generate a file, we use a different number of spaces using our in-ram dictionary.
    /// The static function that does this is UpdateSpaces.
    /// During tests, a single space seems sufficient, but to be safe, I used MaxSpaces constant to use more than one space.
    /// 
    /// This system has some drawback, though:
    ///     1) You need to use this Task, so, you have to include a DLL and add some tags to your MSBuild project.
    ///     2) This should works for full pathed filenames.
    ///     3) Not garbage collector friendly - some objects (internal static dictionary and some Visual Studio cached files) will never be released and there is no way to release them.
    ///        This is a small amount of RAM, though, and should not be a problem during development.
    ///        During command-line MSBuild this problem can be ignored.
    /// </summary>

Original issue reported on code.google.com by Dmitry.G...@gmail.com on 17 Oct 2012 at 5:53