[clang][nullability] Remove `RecordValue`. (#89052)

This class no longer serves any purpose; see also the discussion here:
https://reviews.llvm.org/D155204#inline-1503204

A lot of existing tests in TransferTest.cpp check for the existence of
`RecordValue`s. Some of these checks are now simply redundant and have
been
removed. In other cases, tests were checking for the existence of a
`RecordValue` as a way of testing whether a record has been initialized.
I have
typically changed these test to instead check whether a field of the
record has
a value.