Use the same alignment for writing and readin .swift_ast section contents.

This fixes a bug that made it impossible to read any subsequent Swift modules
out of a .swift_ast section a previous section had a size divisible by 4.

rdar://problem/57110020

lib/Serialization/ModuleFile.cpp

@@ -375,8 +375,8 @@ ValidationInfo serialization::validateSerializedAST(
   ValidationInfo result;
 
   // Check 32-bit alignment.
-  if (data.size() % 4 != 0 ||
-      reinterpret_cast<uintptr_t>(data.data()) % 4 != 0)
+  if (data.size() % SWIFTMODULE_ALIGNMENT != 0 ||
+      reinterpret_cast<uintptr_t>(data.data()) % SWIFTMODULE_ALIGNMENT != 0)
     return result;
 
   llvm::BitstreamCursor cursor(data);