Teach the explicit constructor check about constexpr.

If a constructor is marked constexpr it evades the explicit constructor
check right now, since the check only knows about the inline keyword.
Teach it that constexpr can be used also.

cpplint/cpplint.py

@@ -2779,7 +2779,8 @@ def CheckForNonStandardConstructs(filename, clean_lines, linenum,
   # Look for single-argument constructors that aren't marked explicit.
   # Technically a valid construct, but against style.
   explicit_constructor_match = Match(
-      r'\s+(?:inline\s+)?(explicit\s+)?(?:inline\s+)?%s\s*'
+      r'\s+(?:(?:inline|constexpr)\s+)*(explicit\s+)?'
+      r'(?:(?:inline|constexpr)\s+)*%s\s*'
       r'\(((?:[^()]|\([^()]*\))*)\)'
       % re.escape(base_classname),
       line)