[XamlCodeGenerator] Map errors to XAML line numbers

[Youssef1313]

Current behavior

When an invalid XAML is encountered, it's not always easy to see where the error is.

Expected behavior

The source generator should generate #line directives instead of "comments" that mention line numbers. We should make sure that the current way of generating source information as comments is accurate enough before moving to #line right away.

That way, compile errors in generated code will easily map to XAML

For example:

using System;

class C
{
#line 11 "BlankPage1.xaml"
    code producing error; // the error here will point to Line 11 in BlankPage1.xaml
#line hidden

}