Move Sema's key function around and add more comments

The previous comment was pretty obscure.

Author: rnk

clang/include/clang/Sema/Sema.h

@@ -353,9 +353,6 @@ class Sema final {
   Sema(const Sema &) = delete;
   void operator=(const Sema &) = delete;
 
-  /// A key method to reduce duplicate debug info from Sema.
-  virtual void anchor();
-
   ///Source of additional semantic information.
   ExternalSemaSource *ExternalSource;
 
@@ -1527,6 +1524,13 @@ class Sema final {
   /// initialized but before it parses anything.
   void Initialize();
 
+  /// This virtual key function only exists to limit the emission of debug info
+  /// describing the Sema class. GCC and Clang only emit debug info for a class
+  /// with a vtable when the vtable is emitted. Sema is final and not
+  /// polymorphic, but the debug info size savings are so significant that it is
+  /// worth adding a vtable just to take advantage of this optimization.
+  virtual void anchor();
+
   const LangOptions &getLangOpts() const { return LangOpts; }
   OpenCLOptions &getOpenCLOptions() { return OpenCLFeatures; }
   FPOptions     &getCurFPFeatures() { return CurFPFeatures; }