Recognize metadata property backing fields

[siegfriedpammer]

Summary

• Recognize property backing fields through the existing metadata lookup when CompilerGeneratedAttribute is absent.
• Keep the static/type/name checks before treating a field as a property's backing storage.
• Add an ILPretty regression fixture with a C#-style backing field but no CompilerGeneratedAttribute.

Testing

• OPENSSL_ENABLE_SHA1_SIGNATURES=1 dotnet test ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj --report-trx --filter FullyQualifiedName~ILPrettyTestRunner.PropertyBackingFieldWithoutCompilerGeneratedAttribute
• Top-200 decompdiff: 37,741 types compared, 2 changed, 0 new errors, compiler-generated name leaks 2555 -> 2549

Assisted-by: OpenCode:openai/gpt-5.5:OpenCode