Due to some early experience with this function initially exposing too much
implementation detail that tended to cause readers to get fixated on the
wrong parts of the error message, convert.MismatchMessage switched to
a mode where in some cases it withholds information about what type was
given to a failed type conversion.

However, this moved it to the opposite extreme of being _too_ cautious and
not reporting some information that _is_ more helpful than confusing.

After referring back to my earlier notes on what tended to cause confusion,
this now makes a more thorough effort to classify various "unlikely to
cause confusion" combinations of got and want type and in those cases it
will now report what type was given alongside what value was required.

This is a subjective boundary and so might get negotiated further in
future, but based on the existing tests and some further manual testing
this seems to make a relatively good compromise where it generates useful
information in most cases where the direct type is the most likely problem,
while still avoiding saying confusing things that expose implementation
details like untyped nulls, empty collections with as-yet-unknown element
types, etc.

This work was sponsored by Spacelift as an indirect contribution to the
OpenTofu project. Thanks!