`.sln.metaproj.tmp` written to disk too soon

[This](https://github.com/Microsoft/msbuild/blob/master/src/Build/Construction/Solution/SolutionProjectGenerator.cs#L880-L887) writes the metaproj for the solution to disk here. But after this point there are changes made to the project, eg. a `.csproj.metaproj` reference might be added like in #1957 .
Moving that to around [here](https://github.com/Microsoft/msbuild/blob/master/src/Build/Construction/Solution/SolutionProjectGenerator.cs#L735) fixes it. But I'm a little unsure because of this [comment](https://github.com/Microsoft/msbuild/blob/master/src/Build/Construction/Solution/SolutionProjectGenerator.cs#L880-L881):

```
// For debugging purposes: some information is lost when evaluating into a project instance,
// so make it possible to see what we have at this point.
```