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/LeakFolding.h b/libmemunreachable/LeakFolding.h
index 732d3f2..9c6a525 100644
--- a/libmemunreachable/LeakFolding.h
+++ b/libmemunreachable/LeakFolding.h
@@ -54,7 +54,7 @@
     bool dominator;
     SCCInfo* accumulator;
 
-    SCCInfo(Allocator<SCCInfo> allocator) : node(this, allocator),
+    explicit SCCInfo(Allocator<SCCInfo> allocator) : node(this, allocator),
         count(0), size(0), cuumulative_count(0), cuumulative_size(0),
         dominator(false), accumulator(nullptr) {}
    private: