Events don't require unsafe context for pointers in types

**Version Used**: [5.3.0-2.25619.2](https://dev.azure.com/dnceng/public/_artifacts/feed/dotnet-tools/NuGet/Microsoft.Net.Compilers.Toolset/overview/5.3.0-2.25619.2) ([6a135ea](https://github.com/dotnet/roslyn/commit/6a135ea0ca8b7332d11fa78d68f0a14a65f2644f))

**Steps to Reproduce**:

```cs
class C
{
event System.Action E; // no error
}
```

**Expected Behavior**: "error CS0214: Pointers and fixed size buffers may only be used in an unsafe context" like for example a property would get:

```cs
class C
{
System.Action P { get; set; } // error: pointers may only be used in an unsafe context
}
```

**Actual Behavior**: No errors.

Unsafe errors are intentionally suppressed (see below) but the assumption in the comment is wrong (it is valid to have a pointer inside a delegate and type argument as the example above shows):

https://github.com/dotnet/roslyn/blob/22d4d4d756c5e6e66f463c21b2c4e0e41f3d9839/src/Compilers/CSharp/Portable/Symbols/Source/SourceEventSymbol.cs#L819-L827

Fixing this would be a breaking change.

Also, using the event outside `unsafe` context fails anyway.

Start in src/Compilers/CSharp/Portable/Symbols/Source/SourceEventSymbol.cs around lines 819-827 and reproduce the C# event sample to confirm the current behavior. Trace the existing unsafe-error suppression and locate the compiler regression-test area; done means the event declaration reports the expected unsafe-context diagnostic consistently with the property case, with coverage for the sample.

Written by the indexing model from the issue text.