[Backtracing] Add a precondition to protect against problems in future.

Because the cache uses the page addresses as its keys, we should have
a precondition to check that we haven't passed an address within a page.
The latter would result in confusing behaviour so it's worth diagnosing
up front.

rdar://117681625

Author: al45tair

stdlib/public/Backtracing/CachingMemoryReader.swift

@@ -41,6 +41,8 @@ public class CachingMemoryReader<T: MemoryReader>: MemoryReader {
   }
 
   private func getPage(at address: Address) throws -> UnsafeRawBufferPointer {
+    precondition((address & Address(pageMask)) == 0)
+
     if let page = cache[address] {
       return page
     }