[analyzer][NFC] Require explicit matching mode for CallDescriptions (#92454)

This commit deletes the "simple" constructor of `CallDescription` which
did not require a `CallDescription::Mode` argument and always used the
"wildcard" mode `CDM::Unspecified`.

A few months ago, this vague matching mode was used by many checkers,
which caused bugs like https://github.com/llvm/llvm-project/issues/81597
and https://github.com/llvm/llvm-project/issues/88181. Since then, my
commits improved the available matching modes and ensured that all
checkers explicitly specify the right matching mode.

After those commits, the only remaining references to the "simple"
constructor were some unit tests; this commit updates them to use an
explicitly specified matching mode (often `CDM::SimpleFunc`).

The mode `CDM::Unspecified` was not deleted in this commit because it's
still a reasonable choice in `GenericTaintChecker` and a few unit tests.