Emit CompilerFeatureRequiredAttribute for instance operators

[AlekseyTs]

This prevents old compilers and other compilers (VB, etc.) from consuming the new APIs in an unintended way.

[AlekseyTs]

@cston, @333fred, @dotnet/roslyn-compiler Please review.

[AlekseyTs]

@cston, @333fred, @dotnet/roslyn-compiler Please review.

[cston]

        var comp = CreateCompilation([source, CompilerFeatureRequiredAttribute], targetFramework: TargetFramework.Net90);

Consider not including CompilerFeatureRequiredAttribute source in at least one test that targets .NET 7 or later, so it's clear the BCL definition is sufficient. #Pending

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Refers to: src/Compilers/CSharp/Test/Emit3/Symbols/UserDefinedCompoundAssignmentOperatorsTests.cs:162 in 50a18fd. [](commit_id = 50a18fd, deletion_comment = False)

[AlekseyTs]

@333fred, @dotnet/roslyn-compiler For the second review

[AlekseyTs]

        var comp = CreateCompilation([source, CompilerFeatureRequiredAttribute], targetFramework: TargetFramework.Net90);

Consider not including CompilerFeatureRequiredAttribute source in at least one test that targets .NET 7 or later, so it's clear the BCL definition is sufficient.

I will make a change like that in a different PR.

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In reply to: 2818921287

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Refers to: src/Compilers/CSharp/Test/Emit3/Symbols/UserDefinedCompoundAssignmentOperatorsTests.cs:162 in 50a18fd. [](commit_id = 50a18fd, deletion_comment = False)