The YAML test helpers didn't use the `Printable` abstraction in one
place and instead directly used `<<` with a `std::ostream`. This matches
the `require`s expression in the `error_test_helpers.h` printing logic
for `ErrorOr`, but fails to provide the necessary implementation for
`llvm::formatv` to succeed with the `Yaml::Value` type.
The main fix is to use `Printable` and to define the `Print` method in
terms of `llvm::raw_ostream`. We already have all the mapping hooks in
place to also support `std::ostream` when needed based on that
definition.
This also adds some constraints to the printing in
`error_test_helpers.h` so it is a bit less under-constrained and more
understandable when it is correctly being used. These are just tidying
though, they aren't what makes these headers work together.
I've added a test to try and make sure these test helpers compose as
well.
Chandler Carruth
b2ab53e49c