Avoid BuildHost crash in Mono due to missing types

The LoadProjectFileAsync routine calls EnsureMSBuildLoaded to make
sure the Microsoft.Build.dll is accessible, but requires types from
that assembly within the routine itself.  This may not always work
with Mono, as the JIT may look up the types during compilation.

Fixed by splitting out a LoadProjectFileAsyncCore routine, similarly
to what is already done for GetProjectsInSolution.

Fixes dotnet/runtime#101121.

src/Workspaces/Core/MSBuild.BuildHost/BuildHost.cs

@@ -161,9 +161,15 @@ private void EnsureMSBuildLoaded(string projectFilePath)
     /// <summary>
     /// Returns the target ID of the <see cref="ProjectFile"/> object created for this.
     /// </summary>
-    public async Task<int> LoadProjectFileAsync(string projectFilePath, string languageName, CancellationToken cancellationToken)
+    public Task<int> LoadProjectFileAsync(string projectFilePath, string languageName, CancellationToken cancellationToken)
     {
         EnsureMSBuildLoaded(projectFilePath);
+        return LoadProjectFileAsyncCore(projectFilePath, languageName, cancellationToken);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)] // Do not inline this, since this uses MSBuild types which are being loaded by the caller
+    private async Task<int> LoadProjectFileAsyncCore(string projectFilePath, string languageName, CancellationToken cancellationToken)
+    {
         CreateBuildManager();
 
         ProjectFileLoader projectLoader = languageName switch