Add API Analyzer Assembly (#906)

* Start on API analyzers

* Finish GuildAccessAnalyzer

* Update build script (will this do?)

* Correct slashes

* Extrapolate DerivesFromModuleBase() to an extension method

* Quick refactoring

* Add doc file

test/Discord.Net.Tests/AnalyzerTests/Helpers/DiagnosticResult.cs

@@ -0,0 +1,87 @@
+using Microsoft.CodeAnalysis;
+using System;
+
+namespace TestHelper
+{
+    /// <summary>
+    /// Location where the diagnostic appears, as determined by path, line number, and column number.
+    /// </summary>
+    public struct DiagnosticResultLocation
+    {
+        public DiagnosticResultLocation(string path, int line, int column)
+        {
+            if (line < -1)
+            {
+                throw new ArgumentOutOfRangeException(nameof(line), "line must be >= -1");
+            }
+
+            if (column < -1)
+            {
+                throw new ArgumentOutOfRangeException(nameof(column), "column must be >= -1");
+            }
+
+            this.Path = path;
+            this.Line = line;
+            this.Column = column;
+        }
+
+        public string Path { get; }
+        public int Line { get; }
+        public int Column { get; }
+    }
+
+    /// <summary>
+    /// Struct that stores information about a Diagnostic appearing in a source
+    /// </summary>
+    public struct DiagnosticResult
+    {
+        private DiagnosticResultLocation[] locations;
+
+        public DiagnosticResultLocation[] Locations
+        {
+            get
+            {
+                if (this.locations == null)
+                {
+                    this.locations = new DiagnosticResultLocation[] { };
+                }
+                return this.locations;
+            }
+
+            set
+            {
+                this.locations = value;
+            }
+        }
+
+        public DiagnosticSeverity Severity { get; set; }
+
+        public string Id { get; set; }
+
+        public string Message { get; set; }
+
+        public string Path
+        {
+            get
+            {
+                return this.Locations.Length > 0 ? this.Locations[0].Path : "";
+            }
+        }
+
+        public int Line
+        {
+            get
+            {
+                return this.Locations.Length > 0 ? this.Locations[0].Line : -1;
+            }
+        }
+
+        public int Column
+        {
+            get
+            {
+                return this.Locations.Length > 0 ? this.Locations[0].Column : -1;
+            }
+        }
+    }
+}