[Stdlib] Mark CVaListPointer as @unsafe.

rdar://135604375

Author: mikeash

stdlib/public/core/CTypes.swift

@@ -284,6 +284,7 @@ extension UInt {
 /// A wrapper around a C `va_list` pointer.
 #if arch(arm64) && !(os(macOS) || os(iOS) || os(tvOS) || os(watchOS) || os(visionOS) ||  os(Windows))
 @frozen
+@unsafe
 public struct CVaListPointer {
   @usableFromInline // unsafe-performance
   internal var _value: (__stack: UnsafeMutablePointer<Int>?,
@@ -317,6 +318,7 @@ extension CVaListPointer: CustomDebugStringConvertible {
 #else
 
 @frozen
+@unsafe
 public struct CVaListPointer {
   @usableFromInline // unsafe-performance
   internal var _value: UnsafeMutableRawPointer