Add test case that becomes intractably slow with TypeTransformer::transformSubMap() fix

This was reported as rdar://149315905.

Author: slavapestov

test/SILGen/Inputs/opaque_result_type_slow_other.swift

@@ -0,0 +1,8 @@
+public protocol P {}
+public protocol Q {}
+
+public func wrap<T: P & Q>(_ t: T) -> some P & Q {
+  return t
+}
+
+

test/SILGen/opaque_result_type_slow.swift

@@ -0,0 +1,32 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module %S/Inputs/opaque_result_type_slow_other.swift -emit-module -emit-module-path %t/opaque_result_type_slow_other.swiftmodule -enable-library-evolution
+// RUN: %target-swift-emit-silgen %s -I %t
+
+import opaque_result_type_slow_other
+
+struct G<T: P & Q>: P & Q {}
+
+public func myWrap<T: P & Q>(_ t: T) -> some P & Q {
+  return G<T>()
+}
+
+public func wrapWrap<T: P & Q>(_ t: T) -> some P & Q {
+  return wrap(wrap(myWrap(t)))
+}
+
+public struct S: P, Q {}
+
+// We generate a series of substitution maps where each one contains an opaque archetype,
+// and an abstract conformance of this opaque archetype to a protocol. Each opaque archetype
+// then has a substitution map of the same form, and so on.
+//
+// Transforming each substitution map then visits each opaque archetype twice: once as part
+// of the replacement type, and again as part of the conformance.
+//
+// What saves us is that we would skip the conformance substitution in some cases, and perform
+// a lookup instead. This lookup is now load bearing, because changing it into a substitution
+// makes this program intractably slow.
+//
+// The correct fix is to probably cache substituted opaque GenericEnvironments, or
+// substituted SubstitutionMaps, inside of the InFlightSubstitution.
+let x = wrapWrap(wrapWrap(wrapWrap(wrapWrap(wrapWrap(wrapWrap(wrapWrap(wrapWrap(wrapWrap(S())))))))))