Fix google-explicit-constructor warnings in system/core.

* Declare explicit conversion constructors.
* Add NOLINT for implicit conversion constructors.
* Fix also some misaligned indendations.

Bug: 28341362
Change-Id: Idf911f35923b408d92285cc1a053f382ba08c63e
Test: build with clang-tidy
diff --git a/libmemunreachable/Tarjan.h b/libmemunreachable/Tarjan.h
index d7ecdb9..dcd139a 100644
--- a/libmemunreachable/Tarjan.h
+++ b/libmemunreachable/Tarjan.h
@@ -62,7 +62,7 @@
 template<class T>
 class TarjanAlgorithm {
  public:
-  TarjanAlgorithm(Allocator<void> allocator) : index_(0),
+  explicit TarjanAlgorithm(Allocator<void> allocator) : index_(0),
     stack_(allocator), components_(allocator) {}
 
   void Execute(Graph<T>& graph, SCCList<T>& out);