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/Support/Unix/Memory.inc b/llvm/lib/Support/Unix/Memory.inc
index d703191..f3463e5 100644
--- a/llvm/lib/Support/Unix/Memory.inc
+++ b/llvm/lib/Support/Unix/Memory.inc
@@ -73,7 +73,7 @@
   return PROT_NONE;
 }
 
-} // namespace
+} // anonymous namespace
 
 namespace llvm {
 namespace sys {
@@ -265,7 +265,7 @@
 }
 
 bool Memory::setExecutable (MemoryBlock &M, std::string *ErrMsg) {
-  if (M.Address == 0 || M.Size == 0) return false;
+  if (M.Address == nullptr || M.Size == 0) return false;
   Memory::InvalidateInstructionCache(M.Address, M.Size);
 #if defined(__APPLE__) && (defined(__arm__) || defined(__arm64__))
   kern_return_t kr = vm_protect(mach_task_self(), (vm_address_t)M.Address,