Fix Clang-tidy modernize-use-nullptr and modernize-use-override warnings; other minor fixes.

Differential revision: reviews.llvm.org/D16568

llvm-svn: 258831
diff --git a/llvm/lib/Transforms/Utils/SanitizerStats.cpp b/llvm/lib/Transforms/Utils/SanitizerStats.cpp
index af3df2f..9afd175 100644
--- a/llvm/lib/Transforms/Utils/SanitizerStats.cpp
+++ b/llvm/lib/Transforms/Utils/SanitizerStats.cpp
@@ -27,7 +27,7 @@
   EmptyModuleStatsTy = makeModuleStatsTy();
 
   ModuleStatsGV = new GlobalVariable(*M, EmptyModuleStatsTy, false,
-                                     GlobalValue::InternalLinkage, 0);
+                                     GlobalValue::InternalLinkage, nullptr);
 }
 
 ArrayType *SanitizerStatReport::makeModuleStatsArrayTy() {