[ASTWriter] Do not write ObjCCategories if empty.

This is a fix for a completely unrelated patch, that started to fail the
explicit-build.cpp test because the size of the b.pcm and b-not-a.pcm files
became the same. The alignment added by empty ObjCCategory blobs being written
to the file causes them to be the same size, and the error 'module file has a
different size than expected' will not be emitted.

This prevents that issue by not saving the ObjCCategories if it is empty. The
change in clang/lib/Serialization/ASTReaderDecl.cpp is just a format, but shows
that the only use of ObjCCategoriesMap loaded from the file will be OK with
null (never loaded) data.

Author: davemgreen

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -4634,11 +4634,10 @@ namespace {
 
       // Perform a binary search to find the local redeclarations for this
       // declaration (if any).
-      const ObjCCategoriesInfo Compare = { LocalID, 0 };
-      const ObjCCategoriesInfo *Result
-        = std::lower_bound(M.ObjCCategoriesMap,
-                           M.ObjCCategoriesMap + M.LocalNumObjCCategoriesInMap,
-                           Compare);
+      const ObjCCategoriesInfo Compare = {LocalID, 0};
+      const ObjCCategoriesInfo *Result = std::lower_bound(
+          M.ObjCCategoriesMap,
+          M.ObjCCategoriesMap + M.LocalNumObjCCategoriesInMap, Compare);
       if (Result == M.ObjCCategoriesMap + M.LocalNumObjCCategoriesInMap ||
           LocalID != Result->getDefinitionID()) {
         // We didn't find anything. If the class definition is in this module

clang/lib/Serialization/ASTWriter.cpp

@@ -4978,6 +4978,9 @@ void ASTWriter::WriteCUDAPragmas(Sema &SemaRef) {
 }
 
 void ASTWriter::WriteObjCCategories() {
+  if (ObjCClassesWithCategories.empty())
+    return;
+
   SmallVector<ObjCCategoriesInfo, 2> CategoriesMap;
   RecordData Categories;