[stdlib] Don’t do full integer conversions in _roundUp

This code is exercised on pretty much every access to ManagedBuffer — let’s keep it as simple as possible.

The overflow case is not expected to ever occur here; this code is used to align offsets within objects, it isn’t expected to get inputs anywhere near Int.max.

Author: lorentey

stdlib/public/core/Builtin.swift

@@ -39,7 +39,10 @@ internal func _roundUp(_ offset: UInt, toAlignment alignment: Int) -> UInt {
 @inlinable
 internal func _roundUp(_ offset: Int, toAlignment alignment: Int) -> Int {
   _internalInvariant(offset >= 0)
-  return Int(_roundUpImpl(UInt(bitPattern: offset), toAlignment: alignment))
+  let offset = UInt(bitPattern: offset)
+  let result = Int(bitPattern: _roundUpImpl(offset, toAlignment: alignment))
+  _internalInvariant(result >= 0)
+  return result
 }
 
 /// Returns a tri-state of 0 = no, 1 = yes, 2 = maybe.