Reapplying r246209, which exposed language options to the checkers. This time disable UseNullptrCheck when not compiling in C++ mode, but still allow in C++11 mode since it's likely the user wishes to modernize their code.

llvm-svn: 246298
diff --git a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
index ea3bd950..08d6c98 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
@@ -453,7 +453,11 @@
 }
 
 void UseNullptrCheck::registerMatchers(MatchFinder *Finder) {
-  Finder->addMatcher(makeCastSequenceMatcher(), this);
+  // Only register the matcher for C++. Because this checker is used for
+  // modernization, it is reasonable to run it on any C++ standard with the
+  // assumption the user is trying to modernize their codebase.
+  if (getLangOpts().CPlusPlus)
+    Finder->addMatcher(makeCastSequenceMatcher(), this);
 }
 
 void UseNullptrCheck::check(const MatchFinder::MatchResult &Result) {