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/Semaphore.h b/libmemunreachable/Semaphore.h
index 45e8c81..6bcf4ea 100644
--- a/libmemunreachable/Semaphore.h
+++ b/libmemunreachable/Semaphore.h
@@ -24,7 +24,7 @@
 
 class Semaphore {
  public:
-  Semaphore(int count = 0) : count_(count) {}
+  explicit Semaphore(int count = 0) : count_(count) {}
   ~Semaphore() = default;
 
   void Wait(std::chrono::milliseconds ms) {