Rename initialize(with:count:) to initialize(to:count:). (#3601)

As proposed in SE-0107: UnsafeRawPointer.

"with" is considered a vacuous preposition. "to" implies direction.

validation-test/stdlib/StringSlicesConcurrentAppend.swift

@@ -89,7 +89,7 @@ func sliceConcurrentAppendThread(_ tid: ThreadID) {
 
 StringTestSuite.test("SliceConcurrentAppend") {
   barrierVar = UnsafeMutablePointer(allocatingCapacity: 1)
-  barrierVar!.initialize(with: _stdlib_pthread_barrier_t())
+  barrierVar!.initialize(to: _stdlib_pthread_barrier_t())
   var ret = _stdlib_pthread_barrier_init(barrierVar!, nil, 2)
   expectEqual(0, ret)