Address a bunch of clang-tidy complaints.

There were a bunch more unreasonable/incorrect ones, but these ones
seemed legit. Nothing very interesting, though.

Bug: N/A
Test: ran tests, benchmarks
Change-Id: If66971194d4a7b4bf6d0251bedb88e8cdc88a76f
diff --git a/libc/malloc_debug/MapData.h b/libc/malloc_debug/MapData.h
index 0238139..895f78f 100644
--- a/libc/malloc_debug/MapData.h
+++ b/libc/malloc_debug/MapData.h
@@ -41,7 +41,7 @@
   MapEntry(uintptr_t start, uintptr_t end, uintptr_t offset, const char* name, size_t name_len)
       : start(start), end(end), offset(offset), name(name, name_len) {}
 
-  MapEntry(uintptr_t pc) : start(pc), end(pc) {}
+  explicit MapEntry(uintptr_t pc) : start(pc), end(pc) {}
 
   uintptr_t start;
   uintptr_t end;
diff --git a/libc/malloc_debug/OptionData.h b/libc/malloc_debug/OptionData.h
index 80190f5..5c2d272 100644
--- a/libc/malloc_debug/OptionData.h
+++ b/libc/malloc_debug/OptionData.h
@@ -34,7 +34,7 @@
 
 class OptionData {
  public:
-  OptionData(DebugData* debug) : debug_(debug) {}
+  explicit OptionData(DebugData* debug) : debug_(debug) {}
   ~OptionData() = default;
 
  protected:
diff --git a/libc/malloc_debug/RecordData.h b/libc/malloc_debug/RecordData.h
index 6e19923..ccabac2 100644
--- a/libc/malloc_debug/RecordData.h
+++ b/libc/malloc_debug/RecordData.h
@@ -66,7 +66,7 @@
 
 class AllocEntry : public RecordEntry {
  public:
-  AllocEntry(void* pointer);
+  explicit AllocEntry(void* pointer);
   virtual ~AllocEntry() = default;
 
  protected:
@@ -92,7 +92,7 @@
 
 class FreeEntry : public AllocEntry {
  public:
-  FreeEntry(void* pointer);
+  explicit FreeEntry(void* pointer);
   virtual ~FreeEntry() = default;
 
   std::string GetString() const override;
diff --git a/libc/malloc_debug/TrackData.h b/libc/malloc_debug/TrackData.h
index f7486e9..9a649b9 100644
--- a/libc/malloc_debug/TrackData.h
+++ b/libc/malloc_debug/TrackData.h
@@ -46,7 +46,7 @@
 
 class TrackData : public OptionData {
  public:
-  TrackData(DebugData* debug_data);
+  explicit TrackData(DebugData* debug_data);
   virtual ~TrackData() = default;
 
   void GetList(std::vector<const Header*>* list);