Specify SizeConst when marshalling field as ByValArray

[elinor-fung]

When marshalling using UnmanagedType.ByValArray, the SizeConst field should be specified.

struct MyStruct
{
    [MarshalAs(UnmanagedType.ByValArray)]
    int[] Foo;
}

Category: Interoperability
Default: Enabled

cc @terrajobst @stephentoub @AaronRobinsonMSFT @jkoritzinsky

[AaronRobinsonMSFT]

I think any of these type of requirements should be in some analyzer.

[terrajobst]

• The analyzer should probably also handle UnmanagedType.ByValTStr
  • Should be the same diagnostic ID with a slightly different message
• Assuming code like this wouldn't work at runtime, turning the rule on by default seems fine
  • If code like this works today, then on by default might be noisy

[jkoritzinsky]

Roslyn already requires SizeConst for ByValTStr. Should we instead extend Roslyn to also require SizeConst for ByValArray instead of writing a separate analyzer?