Unblock official build on VS 15.5+

[dagood]

Unblocks the source-indexer build: https://github.com/dotnet/core-eng/issues/2728

Note: MSBuild is frozen to a LKG commit from a few months ago. Its new build system freezes when I run it in this VSTS build, so I'm still looking into that.

Brings in this submodule PR: dotnet/SourceBrowser#4

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Old description:

It would be better to remove this lookup completely (as mentioned in https://github.com/dotnet/core-eng/issues/2728), but this should at least unblock the builds for now by continuing the current runtime type lookup pattern.

I didn't have the exact VS version to try this out against, only a 15.6.0 Preview 3.0 vs. the build's 15.5.180.51428, but it looks like the signature has only had one change after func15:
https://github.com/Microsoft/msbuild/blame/master/src/Shared/FileMatcher.cs#L1692-L1698
https://github.com/Microsoft/msbuild/blame/0c3e87cbbfc1c32c29029e61b985fda2f3714983/src/Shared/FileMatcher.cs#L1402-L1407

[dagood]

@alexperovich Does it make sense to push this branch to the dotnet fork and run the official build on it to test it out?

[alexperovich]

Yea, go ahead and do that to test it. Just make sure to clean up the branch after.

[dagood]

The SelectProjects fix worked, but now there's an MSBuild error running HtmlGenerator.exe:

18:03:29 Reading assembly names from F:\a\_work\3\s\bin\repo\corefxlab\src\System.Azure.Experimental\System.Azure.Experimental.csproj

Unhandled Exception: Microsoft.Build.Exceptions.InvalidProjectFileException: Invalid static method invocation syntax: "[MSBuild]::IsRunningFromVisualStudio()". Method '[MSBuild]::IsRunningFromVisualStudio' not found. Static method invocation should be of the form: $([FullTypeName]::Method()), e.g. $([System.IO.Path]::Combine(`a`, `b`)).   C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets

Poking around tells me we need to update to MSBuild 15.5. I'll work on that in this PR.

[dagood]

Updated the PR with the changes that just made it through the official build, PTAL.

[alexperovich]

LGTM 👍