Currently, formatv erases types using a base class and calls the virtual
function format() to format the objects. To avoid these vtables,
refactor formatv to instead store function_refs to a functor (struct
with overloaded operator()). This saves ~5kiB in vtables.
Additionally, add a static assertion that a formatter is present instead
of relying on errors due to missing templates. This requires a little
change to MLIR's tgfmt to use a different name -- previously, the
substitution failure on ArrayRef<> would cause the variadic function to
be skipped, but a static assertion failure is not a substitution error.
Also, simplify the code in FormatVariadicDetails to use if constexpr
instead of template overloads with enable_if, making the code shorter
and easier to read.
Alexis Engelke
3aec6a40bb