Add support for '--config=clang-tidy' (#3559)

This sets things up to use `bazel` to run `clang-tidy` using
https://github.com/erenon/bazel_clang_tidy.

I'm fixing issues outside of explorer, and disabling clang-tidy for
targets in explorer that have legacy issues. I was going to disable
clang-tidy for targets in explorer such as interpreter anyways, because
they're slow to parse, and just extended that to the currently failing
targets.

common/init_llvm.cpp

@@ -21,7 +21,7 @@ InitLLVM::InitLLVM(int& argc, char**& argv)
   argv = args_.data();
 
   // `argv[argc]` is expected to be a null pointer.
-  args_.push_back(0);
+  args_.push_back(nullptr);
 
   llvm::setBugReportMsg(
       "Please report issues to "