Make it possible to customize MIDL parameters

[abolade]

The problem: MetadataTasks.dll currently uses hard-coded parameters for MIDL compilation. However, certain IDL files require that /notlb be specified as a parameter (e.g. those using system_handle(...) parameters on interface methods), as otherwise midl.exe fails with an error.

The proposed solution: Provide a build directive that allows MIDL parameters to be customized in the CompileIdls task, e.g.:

namespace MetadataTasks
{
    public class CompileIdls : Task
    {
< snip >
        public string MidlSwitches
        {
            get; set;
        }

[BenJKuhn]

I only have a cursory understanding of TLBs. For our purposes, is there any reason we can't just always pass /notlb? Are there IDLs that require that we generate them? Allowing customization seems like a reasonable plan either way.

[mikebattista]

See #1432 (comment) for PR and comment on changing the default.

[mikebattista]

We set /notlb by default now with the above commit. I also added a MidlSwitches property to allow adding additional parameters to see if that will be useful for other scenarios.