Replace nominal type descriptors with a hierarchy of context descriptors.

This new format more efficiently represents existing information, while
more accurately encoding important information about nested generic
contexts with same-type and layout constraints that need to be evaluated
at runtime. It's also designed with an eye to forward- and
backward-compatible expansion for ABI stability with future Swift
versions.

test/stdlib/Mirror.swift

@@ -723,7 +723,7 @@ mirrors.test("PlaygroundQuickLook") {
   case .text(let text):
 #if _runtime(_ObjC)
 // FIXME: Enable if non-objc hasSuffix is implemented.
-    expectTrue(text.contains("X #1 in"), text)
+    expectTrue(text.contains(").X"), text)
 #endif
   default:
     expectTrue(false)