WebcilReader: copy updated struct for BlobReader.ReadUtf8NullTerminated

When using reflection to call a method on BlobReader, make sure to
copy the struct after the call since the internal state is modified

Fixes PdbChecksum extraction from Webcil files

(This was a problem even before webcil-in-wasm)

src/libraries/Microsoft.NET.WebAssembly.Webcil/src/Webcil/WebcilReader.Reflection.cs

@@ -29,7 +29,13 @@ internal static class Reflection
             return mi;
         });
 
-        internal static string? ReadUtf8NullTerminated(BlobReader reader) => (string?)s_readUtf8NullTerminated.Value.Invoke(reader, null);
+        internal static string? ReadUtf8NullTerminated(ref BlobReader reader)
+        {
+            object boxedReader = reader;
+            string? result = (string?)s_readUtf8NullTerminated.Value.Invoke(boxedReader, null);
+            reader = (BlobReader) boxedReader; // the call modifies the struct state, make sure to copy it back.
+            return result;
+        }
 
         private static readonly Lazy<ConstructorInfo> s_codeViewDebugDirectoryDataCtor = new Lazy<ConstructorInfo>(() =>
         {

src/libraries/Microsoft.NET.WebAssembly.Webcil/src/Webcil/WebcilReader.cs

@@ -220,12 +220,12 @@ private static CodeViewDebugDirectoryData DecodeCodeViewDebugDirectoryData(BlobR
 
         Guid guid = reader.ReadGuid();
         int age = reader.ReadInt32();
-        string path = ReadUtf8NullTerminated(reader)!;
+        string path = ReadUtf8NullTerminated(ref reader)!;
 
         return MakeCodeViewDebugDirectoryData(guid, age, path);
     }
 
-    private static string? ReadUtf8NullTerminated(BlobReader reader) => Reflection.ReadUtf8NullTerminated(reader);
+    private static string? ReadUtf8NullTerminated(ref BlobReader reader) => Reflection.ReadUtf8NullTerminated(ref reader);
 
     private static CodeViewDebugDirectoryData MakeCodeViewDebugDirectoryData(Guid guid, int age, string path) => Reflection.MakeCodeViewDebugDirectoryData(guid, age, path);
 
@@ -316,7 +316,7 @@ public PdbChecksumDebugDirectoryData ReadPdbChecksumDebugDirectoryData(DebugDire
 
     private static PdbChecksumDebugDirectoryData DecodePdbChecksumDebugDirectoryData(BlobReader reader)
     {
-        var algorithmName = ReadUtf8NullTerminated(reader);
+        var algorithmName = ReadUtf8NullTerminated(ref reader);
         byte[]? checksum = reader.ReadBytes(reader.RemainingBytes);
         if (string.IsNullOrEmpty(algorithmName) || checksum == null || checksum.Length == 0)
         {