Reduce LOH allocations in MetadataWriter (#78245)

MetadataWriter's use of Dictionary leads to LOH allocations in the Roslyn CA process. Instead, switch over to SegmentedDictionary to reduce that pressure.

From the Roslyn C# completion speedometer test, about 16% of all LOH allocations occur adding to MetadataWriter._signatureIndex. This type represents the 2nd highest entry on the LOH list.

After changing to a SegmentedDictionary these LOH are no longer present.

See PR for images depicting the before/after allocations.

Author: ToddGrun

src/Compilers/Core/Portable/PEWriter/MetadataWriter.cs

@@ -111,7 +111,7 @@ protected MetadataWriter(
 
             // EDMAURER provide some reasonable size estimates for these that will avoid
             // much of the reallocation that would occur when growing these from empty.
-            _signatureIndex = new Dictionary<ISignature, KeyValuePair<BlobHandle, ImmutableArray<byte>>>(module.HintNumberOfMethodDefinitions, ReferenceEqualityComparer.Instance); //ignores field signatures
+            _signatureIndex = new SegmentedDictionary<ISignature, KeyValuePair<BlobHandle, ImmutableArray<byte>>>(module.HintNumberOfMethodDefinitions, ReferenceEqualityComparer.Instance); //ignores field signatures
 
             this.Context = context;
             this.messageProvider = messageProvider;
@@ -443,7 +443,7 @@ private bool IsMinimalDelta
         private readonly Dictionary<IFieldReference, BlobHandle> _fieldSignatureIndex = new Dictionary<IFieldReference, BlobHandle>(ReferenceEqualityComparer.Instance);
 
         // We need to keep track of both the index of the signature and the actual blob to support VB static local naming scheme.
-        private readonly Dictionary<ISignature, KeyValuePair<BlobHandle, ImmutableArray<byte>>> _signatureIndex;
+        private readonly SegmentedDictionary<ISignature, KeyValuePair<BlobHandle, ImmutableArray<byte>>> _signatureIndex;
 
         private readonly Dictionary<IMarshallingInformation, BlobHandle> _marshallingDescriptorIndex = new Dictionary<IMarshallingInformation, BlobHandle>();
         protected readonly List<MethodImplementation> methodImplList = new List<MethodImplementation>();