struct PrintfType

A field.h type will implicitly convert to its underlying integer type, which

is is why diagnostics-ostream.h doesn't need to define operator

<

<

for them.

But the generic definition (for uint64_t and equivalents) and the

specializations for integer types intentionally don't work as template

instantiations for just any type convertible to their respective integer

types, because we don't want to use them for signed integer types.