'Insert arguments on Tab' should not consider captured primary constructor parameters

**Version Used**:
VS 17.10.3

**Steps to Reproduce**:
```cs
class MyClass(int i)
{
private readonly int _i = i;

void M()
{
N$$
}

void N(int a)
{
}
}
```
Make sure you have the feature enabled.
Press `Tab` at `$$` to insert arguments for method.

**Expected Behavior**:
`i` is ignored since it is a captured parameter, so I get `N(_i)`

**Actual Behavior**:
Inserted argument is `i`, so after applying the feature I immediately get a compiler warning about double storage

The issue names no implementation file or test. Start by locating the C# “Insert arguments on Tab” feature and reproduce the primary-constructor example; done means Tab chooses `_i` rather than captured parameter `i` without the double-storage warning.

Written by the indexing model from the issue text.