Fix build events buttons even more correctly (#3399)

* Correctly fetch the IVsBuildMacroInfo to use for the build events page

This code is blindly copied and pasted from https://github.com/dotnet/project-system/blob/88606ba869a29aa24979e0d14a199b2430f31f43/src/Microsoft.VisualStudio.Editors/PropPages/BuildEventsPropPage.vb#L162-L176
I will fully admit I have no idea how this is supposed to work.

* Even more correctly fetch the IVsBuildMacroInfo to use for the build events page

My previous change broke projects with the old project system. Try
the old path and if that doesn't work, then try the new path.

Author: jasonmalinowski

vsintegration/src/FSharp.ProjectSystem.PropertyPages/PropertyPages/BuildEventsPropPage.vb

@@ -336,7 +336,22 @@ Namespace Microsoft.VisualStudio.Editors.PropertyPages
             Dim MacroEval As IVsBuildMacroInfo
             Dim MacroValue As String = Nothing
 
-            MacroEval = CType(m_Objects(0), IVsBuildMacroInfo)
+            ' The old project system provides IVsBuildMacroInfo this way...
+            MacroEval = TryCast(m_Objects(0), IVsBuildMacroInfo)
+            If MacroEval Is Nothing Then
+                Dim Hier As IVsHierarchy = Nothing
+                Dim ItemId As UInteger
+                Dim ThisObj As Object = m_Objects(0)
+
+                ' ...whereas CPS requires us to go through IVsBrowseObject.
+                If TypeOf ThisObj Is IVsBrowseObject Then
+                    VSErrorHandler.ThrowOnFailure(CType(ThisObj, IVsBrowseObject).GetProjectItem(Hier, ItemId))
+                ElseIf TypeOf ThisObj Is IVsCfgBrowseObject Then
+                    VSErrorHandler.ThrowOnFailure(CType(ThisObj, IVsCfgBrowseObject).GetProjectItem(Hier, ItemId))
+                End If
+                MacroEval = CType(Hier, IVsBuildMacroInfo)
+            End If
+
             VSErrorHandler.ThrowOnFailure(MacroEval.GetBuildMacroValue(MacroName, MacroValue))
 
             Return MacroValue