Suppressing warnings through project properties UI adds tfm condition in csproj file for multi targeting project

## Repro steps:
1. Create a new NetCore 2.0 Console App
2. Multi-target to netcoreapp2.0;net461:
` netcoreapp2.0;net461`
3. Add a NuGet reference to a net 461 compat package like RestSharp
4. Build -> Show 2 warnings which seems to be another bug: https://github.com/dotnet/sdk/issues/1474
5. Suppress warning NU1701 from Project->Properties->Build->Suppress Warnings
6. Edit the csproj file -

```

Exe
netcoreapp2.0;net461

1701;1702;1705;NU1701

```

The issue here is that the user did not select `netcoreapp2.0` while suppressing the warning but the csproj has `netcoreapp2.0` in the condition.

## Other Info:

if you try the same steps in a project that does not multi-target then the condition is fine -

```

Exe
netcoreapp2.0

1701;1702;1705;NU1701

```

VS Version: 15.4.0 Preview 2 | d15rel/16828.0

cc: @natidea @emgarten