Enable net46 builds in source-build

[crummel]

End-to-end, this enables VS Code usage with source-build .NET Core (#125 -> dotnet/sdk#2249)

[crummel]

Related: dotnet/sdk#2309, #588.

[dleeapho]

@crummel Please test the E2E scenario with VS Code once PRs are merged.

[dagood]

Merged in #598. I'm going to try validating VS Code/Omnisharp with a local build I'm building of #610.

[dagood]

It works! 😄 (Fedora 28)

I extracted my source-built CLI, ran dotnet new console, then code . to make sure I see a "bad" status. The C# extension tells me it can't load the project. I add the extracted CLI to PATH, run code . again, and I get intellisense and F5 debugging.

[crummel]

Confirmed, I also get a working VS Code. dotnet --info output:

.NET Core SDK (reflecting any global.json):
 Version:   2.1.301
 Commit:    59524873d6

Runtime Environment:
 OS Name:     rhel
 OS Version:  7
 OS Platform: Linux
 RID:         rhel.7-x64
 Base Path:   /home/chris/dotnet/sdk/2.1.301/

Host (useful for support):
  Version: 2.1.1
  Commit:  N/A

.NET Core SDKs installed:
  2.1.301 [/home/chris/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.NETCore.App 2.1.1 [/home/chris/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download