Saving a CPS multi-targeting project unglobbed net45 subtypes

[clairernovotny]

Look at this commit:
clairernovotny/microsoft-authentication-library-for-dotnet@f8880e6

All I did was change net45 to be first in the order but then VS expanded a bunch of crapL
clairernovotny/microsoft-authentication-library-for-dotnet@f8880e6#diff-9abb1283a16cd21deac528e4738327f7R147 by adding the <SubType>Form</SubType> elements. I don't want those in my glob.

It's worse than that too as it put those elements in a top-level item group without any conditions on it. None of the other TFM's should even see that file, that's wrong.

[davkean]

Not sure how this even happened, we haven't even implemented sub type support.

[srivatsn]

I also saw this while changing this repo to dogfood - I think this is Roslyn. Roslyn has a background analyzer which goes through all files to see if they have types with the DesignerAttribute. If they do, then it adds the SubType metadata to the file to mark it for opening with the designer on a double-click instead of opening the cs file.

That analyzer is such a drain on perf for minimal benefit. I believe @heejaechang is planning to move that analyzer out of proc but it'll be good if we can come up with some better way of representing this information. Leaving this in 15.3 to brainstorm on a path forward.

[tmeschter]

The background analyzer for DesignerAttribute support has been disabled for CPS-based projects--the project system doesn't fully support these kinds of designers, anyway, so adding the metadata isn't really useful.

That being said, we're going to have to support them eventually, and we'll need to deal with this.

Moving out to 15.6.