ItemGroup item condition with metadata doesn't work in Project

`ItemGroup` with an item which has a condition with item metadata doesn't work in `Project`, but it works in a `Target`.

### Steps to reproduce

#### Example `ItemGroup`
```xml

```

This works in a `Target`, but not in `Project`.

#### Command line
```
msbuild
```
or
```
dotnet build
```

### Expected behavior
It uses the metadata to evaluate the condition.

### Actual behavior
It throws an exception:
```
error MSB4191: The reference to custom metadata "X" at position 1 is not allowed in this condition "'%(Content.X)' == 'abc'".
```

### Environment data
`msbuild /version` output:
15.7.179.6572

OS info:
Windows 10

If applicable, version of the tool that invokes MSBuild (Visual Studio, dotnet CLI, etc):
- Visual Studio: 15.7.4.
- .NET CLI: 2.1.300.