[GSB] Generalize rewrite paths to support “absolute” paths.

Previously, the term rewriting logic for same-type constraints was only 
able to express “relative” rewrites, where the base of the type being 
rewritten (i.e., the generic type at the root) is unchanged. This meant
that we were unable to capture same-type constraints such as “T == U” or
“T.Foo == T” within the rewriting system.

Separate out the notion of a rewrite path and extend it with an optional
new base. When we’re simplifying a term and we encounter one of these
replacements, the whole type from the base up through the last-matched
path component gets replaced with the replacement path.

Author: DougGregor

include/swift/AST/GenericSignatureBuilder.h

@@ -809,21 +809,11 @@ class GenericSignatureBuilder {
   /// Determine whether the two given types are in the same equivalence class.
   bool areInSameEquivalenceClass(Type type1, Type type2);
 
-  /// Simplify the given type, which is described by a base parameter
-  /// followed by a sequence of associated types.
-  ///
-  /// \param path is updated to describe the best path from the given base
-  /// to the same equivalence class.
-  ///
-  /// \returns true if the path was simplified at all.
-  bool simplifyType(GenericParamKey base,
-                    SmallVectorImpl<AssociatedTypeDecl *> &path);
-
-  /// Simplify the given type down to its anchor.
+  /// Simplify the given dependent type down to its canonical representation.
   ///
   /// \returns null if the type involved dependent member types that
   /// don't have associated types.
-  Type simplifyType(Type type);
+  Type getCanonicalTypeParameter(Type type);
 
   /// Verify the correctness of the given generic signature.
   ///