Fix "Introduce local" generating invalid code for function pointer types

[Copilot]

Fixes the "Introduce local" refactoring generating invalid code for function pointer types by using the type's display string as the variable name (e.g., delegate*<void> delegate*<void> = field;).

Changes

• ITypeSymbolExtensions.CreateParameterName: Added case for IFunctionPointerTypeSymbol to return default parameter name "value" instead of type display string
• Pointer handling preserved: Normal pointers like Customer* continue to unwrap to the pointed-at type to generate meaningful names (e.g., Customer* → customer)
• Explanatory comments: Added "why, not what" comments explaining the different handling for normal pointers vs function pointers
• Tests: Added TestFunctionPointerType and TestPointerType covering both function pointers and custom pointer types

Example

Before:

// Function pointers
delegate*<void> delegate*<void> = field;  // Invalid syntax
return delegate*<void>;

After:

// Function pointers
delegate*<void> value = field;
return value;

// Normal pointers (preserved behavior)
Customer* customer = field;
return customer;

Fixes #80982

Original prompt

---

This section details on the original issue you should resolve

<issue_title>"Introduce local" generates invalid code for function pointer types</issue_title>
<issue_description>Version Used: VS 18.0.0 Insiders [11123.170]

Steps to Reproduce:
Invoke the lightbulb menu at the marked location and select "Introduce local for 'field'".

unsafe class C
{
    public delegate* managed<void> D
    {
        get
        {
            return field$$;
        }
    }
}

Expected Behavior:
Code similar to this is generated:

public delegate* managed<void> D
{
    get
    {
        delegate*<void> v = field;
        return v;
    }
}

Actual Behavior:
This code is generated, which is syntactically invalid (it appears as if the type name is chosen as the name of the local):

public delegate* managed<void> D
{
    get
    {
        delegate*<void> delegate*<void> = field;
        return delegate*<void>;
    }
}

Comments on the Issue (you are @copilot in this section)

• Fixes "Introduce local" generates invalid code for function pointer types #80982

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[CyrusNajmabadi]

@copilot check normal pointers as well.

[Copilot]

@copilot check normal pointers as well.

Done in d860c92. Normal pointer types now return "value" instead of unwrapping to the pointed-at type (which could be keywords like "void" or "int"). Added test coverage for void*.

[CyrusNajmabadi]

@copilot undo the change to pointers. If the user has Customer* we want to still make that as if it was just Customer. Add tests for this as well and add a 'why, not what's comment for this in the code

[Copilot]

Done in b81b069. Restored pointer unwrapping so Customer* generates "customer". Added "why" comments explaining that normal pointers unwrap to get meaningful names while function pointers use the default. Updated test to use Customer* → customer.