Add SuppressLinkerWarningAttribute

[MichalStrehovsky]

This is in support of dotnet/linker#988 - see the spec there for motivation.

We need a way to selectively suppress warnings about linker-unfriendly patterns.

This attribute is largely inspired by SuppressMessageAttribute (that unfortunately cannot be used because it's marked [Conditional] under a condition that we wouldn't expect to enable when the linker runs).

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(
     AttributeTargets.All,
     Inherited = false,
     AllowMultiple = true
     )
    ]
    public sealed class SuppressLinkerWarningAttribute : Attribute
    {
        public SuppressLinkerWarningAttribute(string category, string checkId)
        {
            Category = category;
            CheckId = checkId;
        }

        public string Category { get; }
        public string CheckId { get; }
        public string? MessageId { get; set; }
        public string? Justification { get; set; }
    }
}

[MichalStrehovsky]

Cc @vitek-karas

[terrajobst]

Video

• We should position it as a peer to SuppressMessageAttribute, UnconditionalSuppressMessageAttribute
• It should be System.Diagnostics.CodeAnalysis
• The behavior is the same, except it's not marked [Conditonal] and thus will always remain in metadata
• We considered establishing an inheritance relationshop with SuppressMessageAttribute but concluded we don't need it (neither for docs nor for consumption)

namespace System.Diagnostics.CodeAnalysis
{
    [AttributeUsage(AttributeTargets.All, Inherited = false, AllowMultiple = true)]
    public sealed class UnconditionalSuppressMessageAttribute : Attribute
    {
    	// Same API as SuppressMessageAttribute
    }
}