[clang-tidy] Use default to initialize constructors

Found with modernize-use-equals-default

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/Lexer.cpp

@@ -50,9 +50,7 @@ Lexer::Lexer (const std::string& text)
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-Lexer::~Lexer ()
-{
-}
+Lexer::~Lexer () = default;
 
 ////////////////////////////////////////////////////////////////////////////////
 // When a Lexer object is constructed with a string, this method walks through