Improve Assert output

Improve Assert output

If an assertion expression contained a macro, the failed assertion
message would print the expanded macro, which is usually unhelpful and
confusing.  Restructure the Assert macros to not expand any macros
when constructing the failure message.

This also fixes that the existing output for Assert et al. shows
the *inverted* condition, which is also confusing and not how
assertions usually work.

Discussion: https://www.postgresql.org/message-id/flat/6c68efe3-117a-dcc1-73d4-18ba1ec532e2%402ndquadrant.com