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;