IDE: Use GenericSignatures and interface types (mostly)

There was a ton of complicated logic here to work around
two problems:

- Same-type constraints were not represented properly in
  RequirementReprs, requiring us to store them in strong form
  and parse them out when printing type interfaces.

- The TypeBase::getAllGenericArgs() method did not do the
  right thing for members of protocols and protocol extensions,
  and so instead of simple calls to Type::subst(), we had
  an elaborate 'ArchetypeTransformer' abstraction repeated
  in two places.

Rewrite this code to use GenericSignatures and
GenericFunctionType instead of old-school GenericParamLists
and PolymorphicFunctionType.

This changes the code completion and AST printer output
slightly. A few of the changes are actually fixes for cases
where the old code didn't handle substitutions properly.
A few others are subjective, for example a generic parameter
list of the form <T : Proto> now prints as <T where T : Proto>.

We can add heuristics to make the output whatever we want
here; the important thing is that now we're using modern
abstractions.

lib/IDE/ModuleInterfacePrinting.cpp

@@ -168,7 +168,7 @@ printTypeInterface(ModuleDecl *M, Type Ty, ASTPrinter &Printer,
   }
   Ty = Ty->getRValueType();
   if (auto ND = Ty->getNominalOrBoundGenericNominal()) {
-    PrintOptions Options = PrintOptions::printTypeInterface(Ty.getPointer(), M);
+    PrintOptions Options = PrintOptions::printTypeInterface(Ty.getPointer());
     ND->print(Printer, Options);
     printTypeNameToString(Ty, TypeName);
     return false;
@@ -574,8 +574,7 @@ void swift::ide::printSubmoduleInterface(
                                         Decls.back().first == ET.first, true};
                 if (ET.second)
                   AdjustedOptions.
-                    initArchetypeTransformerForSynthesizedExtensions(NTD,
-                                                               pAnalyzer.get());
+                    initArchetypeTransformerForSynthesizedExtensions(NTD);
                 ET.first->print(Printer, AdjustedOptions);
                 if (ET.second)
                   AdjustedOptions.
@@ -607,8 +606,7 @@ void swift::ide::printSubmoduleInterface(
                   Printer << "\n";
                 if (ET.second)
                   AdjustedOptions.
-                    initArchetypeTransformerForSynthesizedExtensions(NTD,
-                                                               pAnalyzer.get());
+                    initArchetypeTransformerForSynthesizedExtensions(NTD);
                 ET.first->print(Printer, AdjustedOptions);
                 if (ET.second)
                   AdjustedOptions.