Skip parsing types in ILLinkTrim.Descriptors if not needed (#79803)

We should not even be using this parser (#79802). It's surprisingly slow to use this.

This saves about 10% of wallclock time when compiling Hello World (yes, I can't believe my measurements either). We parse the descriptors for the purposes of manifest resources pretty late in the single threaded phase.

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ManifestResourceBlockingPolicy.cs

@@ -109,6 +109,8 @@ private SubstitutionsReader(TypeSystemContext context, XmlReader reader, ModuleD
             {
             }
 
+            protected override bool ShouldProcessTypes => false;
+
             public static HashSet<string> GetSubstitutions(TypeSystemContext context, XmlReader reader, ModuleDesc module, IReadOnlyDictionary<string, bool> featureSwitchValues)
             {
                 var rdr = new SubstitutionsReader(context, reader, module, featureSwitchValues);

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ProcessXmlBase.cs

@@ -129,9 +129,11 @@ private void ProcessAssembly(ModuleDesc assembly)
             _reader.ReadEndElement();
         }
 
+        protected virtual bool ShouldProcessTypes => true;
+
         private void ProcessType(ModuleDesc assembly)
         {
-            if (ShouldProcessElement())
+            if (ShouldProcessTypes && ShouldProcessElement())
             {
                 string typeName = _reader.GetAttribute("fullname");