Make PowerOf2's constructor private.

Ban conversion from integers to PowerOf2 even if explicit
to make all places we create PowerOf2 instances visible.

llvm-svn: 233243
diff --git a/lld/lib/ReaderWriter/MachO/MachONormalizedFile.h b/lld/lib/ReaderWriter/MachO/MachONormalizedFile.h
index c1db36c..940ea63 100644
--- a/lld/lib/ReaderWriter/MachO/MachONormalizedFile.h
+++ b/lld/lib/ReaderWriter/MachO/MachONormalizedFile.h
@@ -108,7 +108,7 @@
 /// can support either kind.
 struct Section {
   Section() : type(llvm::MachO::S_REGULAR),
-              attributes(0), alignment(0), address(0) { }
+              attributes(0), alignment(PowerOf2::create(0)), address(0) { }
 
   StringRef       segmentName;
   StringRef       sectionName;