Update ICSharpCode.Decompiler to 9.1.0.7988 (#78270)

@dibarbet referring to
#71837 (comment)

![image](https://github.com/user-attachments/assets/a4a965d7-794a-40b7-8b9c-b3d989d7f952)

There were no surprises in
https://github.com/icsharpcode/ILSpy/releases/tag/v9.0, and we released
https://github.com/icsharpcode/ILSpy/releases/tag/v9.1 with a couple
more fixes. Now is a good time to update to the 9 series of the
decompiler.

Author: dibarbet

eng/Directory.Packages.props

@@ -230,7 +230,7 @@
     <PackageVersion Include="SQLitePCLRaw.provider.e_sqlite3.net45" Version="$(SqliteVersion)" />
 
     <PackageVersion Include="Humanizer.Core" Version="$(HumanizerVersion)" />
-    <PackageVersion Include="ICSharpCode.Decompiler" Version="8.2.0.7535" />
+    <PackageVersion Include="ICSharpCode.Decompiler" Version="9.1.0.7988" />
     <PackageVersion Include="Microsoft.CodeAnalysis.Elfie" Version="1.0.0" />
 
     <!--

src/EditorFeatures/Test/MetadataAsSource/AbstractMetadataAsSourceTests.cs

@@ -21,7 +21,7 @@ namespace Microsoft.CodeAnalysis.Editor.UnitTests.MetadataAsSource;
 [UseExportProvider]
 public abstract partial class AbstractMetadataAsSourceTests : IAsyncLifetime
 {
-    protected static readonly string ICSharpCodeDecompilerVersion = "8.2.0.7535";
+    protected static readonly string ICSharpCodeDecompilerVersion = "9.1.0.7988";
 
     public virtual Task InitializeAsync()
     {

src/LanguageServer/Protocol/Features/DecompiledSource/AssemblyResolver.cs

@@ -49,12 +49,12 @@ void BuildReferenceCache()
         }
     }
 
-    public Task<PEFile> ResolveAsync(IAssemblyReference name)
+    public Task<MetadataFile> ResolveAsync(IAssemblyReference name)
     {
         return Task.FromResult(Resolve(name));
     }
 
-    public Task<PEFile> ResolveModuleAsync(PEFile mainModule, string moduleName)
+    public Task<MetadataFile> ResolveModuleAsync(MetadataFile mainModule, string moduleName)
     {
         return Task.FromResult(ResolveModule(mainModule, moduleName));
     }
@@ -70,7 +70,7 @@ public PEFile TryResolve(MetadataReference metadataReference, PEStreamOptions st
         return null;
     }
 
-    public PEFile Resolve(IAssemblyReference name)
+    public MetadataFile Resolve(IAssemblyReference name)
     {
         Log("------------------");
         Log(FeaturesResources.Resolve_0, name.FullName);
@@ -147,7 +147,7 @@ PEFile MakePEFile(IAssemblySymbol assembly)
         }
     }
 
-    public PEFile ResolveModule(PEFile mainModule, string moduleName)
+    public MetadataFile ResolveModule(MetadataFile mainModule, string moduleName)
     {
         Log("-------------");
         Log(FeaturesResources.Resolve_module_0_of_1, moduleName, mainModule.FullName);