AST: Introduce abstraction for extension/type decl inheritance clauses.

Wrap the `InheritedEntry` array available on both `ExtensionDecl` and
`TypeDecl` in a new `InheritedTypes` class. This class will provide shared
conveniences for working with inherited type clauses. NFC.

lib/Sema/CSDiagnostics.cpp

@@ -3399,7 +3399,7 @@ bool ContextualFailure::tryProtocolConformanceFixIt(
   auto conformanceDiag =
       emitDiagnostic(diag::assign_protocol_conformance_fix_it, constraint,
                      nominal->getDescriptiveKind(), fromType);
-  if (nominal->getInherited().size() > 0) {
+  if (!nominal->getInherited().empty()) {
     auto lastInherited = nominal->getInherited().back().getLoc();
     auto lastInheritedEndLoc =
         Lexer::getLocForEndOfToken(getASTContext().SourceMgr, lastInherited);