[clang-tidy] Fold the meat of the UseNullPtrCheck into an anonymous namespace.

While convenient, RecursiveASTVisitor generates a ridiculous amount of dead
template code. Making it not visible from the outside lets the compiler
eliminate some of it, shrinking clang-tidy by ~140k.

llvm-svn: 245548
diff --git a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
index 8a0a371..6c3435c 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
@@ -20,6 +20,7 @@
 namespace clang {
 namespace tidy {
 namespace modernize {
+namespace {
 
 const char CastSequence[] = "sequence";
 
@@ -438,6 +439,7 @@
   Expr *FirstSubExpr;
   bool PruneSubtree;
 };
+} // namespace
 
 UseNullptrCheck::UseNullptrCheck(StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context),