Define make_dynamic_error_code(const char *).

The function took either StringRef or Twine. Since string literals are
ambiguous when resolving the overloading, many code calls used this
function with explicit type conversion. That led awkward code like
make_dynamic_error_code(Twine("Error occurred")).

This patch adds a function definition for string literals, so that
you can directly call the function with literals.

llvm-svn: 234841