[Foundation] Add @usableFromInline to MeasurementBridgeType for now

Work around a missing feature in textual interfaces by marking this
convenience typealias as @usableFromInline. Since typealiases don't
actually have a run-time presence, we can still take this out at any
time.

Author: jrose-apple

stdlib/public/SDK/Foundation/Measurement.swift

@@ -206,9 +206,15 @@ extension Measurement {
 
 // Implementation note: similar to NSArray, NSDictionary, etc., NSMeasurement's import as an ObjC generic type is suppressed by the importer. Eventually we will need a more general purpose mechanism to correctly import generic types.
 
+// FIXME: Remove @usableFromInline from MeasurementBridgeType once
+// rdar://problem/44662501 is fixed. (The Radar basically just says "look
+// through typealiases and inherited protocols when printing extensions".)
+
 #if DEPLOYMENT_RUNTIME_SWIFT
+@usableFromInline
 internal typealias MeasurementBridgeType = _ObjectTypeBridgeable
 #else
+@usableFromInline
 internal typealias MeasurementBridgeType = _ObjectiveCBridgeable
 #endif