Extract common lexer code into helpers (#79214)

Author: CyrusNajmabadi

src/Compilers/CSharp/Portable/Parser/AbstractLexer.cs

@@ -4,8 +4,6 @@
 
 using System;
 using System.Collections.Generic;
-using Microsoft.CodeAnalysis.CSharp.Symbols;
-using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Text;
 
 namespace Microsoft.CodeAnalysis.CSharp.Syntax.InternalSyntax
@@ -21,6 +19,8 @@ protected AbstractLexer(SourceText text)
             this.TextWindow = new SlidingTextWindow(text);
         }
 
+        protected int LexemeStartPosition => this.TextWindow.LexemeStartPosition;
+
         public virtual void Dispose()
         {
             this.TextWindow.Dispose();
@@ -131,9 +131,18 @@ protected XmlSyntaxDiagnosticInfo MakeError(int position, int width, XmlParseErr
 
         private int GetLexemeOffsetFromPosition(int position)
         {
-            return position >= TextWindow.LexemeStartPosition ? position - TextWindow.LexemeStartPosition : position;
+            return position >= LexemeStartPosition ? position - LexemeStartPosition : position;
         }
 
+        protected string GetNonInternedLexemeText()
+            => TextWindow.GetText(intern: false);
+
+        protected string GetInternedLexemeText()
+            => TextWindow.GetText(intern: true);
+
+        protected int CurrentLexemeWidth
+            => this.TextWindow.Position - LexemeStartPosition;
+
         protected static SyntaxDiagnosticInfo MakeError(ErrorCode code)
         {
             return new SyntaxDiagnosticInfo(code);