Embedded: Suppress spurious unused result warnings.

The results of `swift_retain(object:)` and `swift_bridgeObjectRetain(object:)`
should be discardable since it's typcial to not need the pointer returned from
these calls. Suppresses unused result warnings that were generated previously.

Author: tshortli

stdlib/public/core/EmbeddedRuntime.swift

@@ -307,6 +307,7 @@ func swift_isUniquelyReferenced_nonNull_native(object: UnsafeMutablePointer<Heap
 }
 
 @_cdecl("swift_retain")
+@discardableResult
 public func swift_retain(object: Builtin.RawPointer) -> Builtin.RawPointer {
   if !isValidPointerForNativeRetain(object: object) { return object }
 
@@ -335,6 +336,7 @@ func swift_retain_n_(object: UnsafeMutablePointer<HeapObject>, n: UInt32) -> Uns
 }
 
 @_cdecl("swift_bridgeObjectRetain")
+@discardableResult
 public func swift_bridgeObjectRetain(object: Builtin.RawPointer) -> Builtin.RawPointer {
   return swift_bridgeObjectRetain_n(object: object, n: 1)
 }