Split -Wcast-function-type into a separate group (#86131)

We want to add -Wcast-function-type to -Wextra (as done in
1de7e6c8), but we do not want to add
-Wcast-function-type-strict in at the same time
(https://lab.llvm.org/buildbot/#/builders/57/builds/33601/steps/5/logs/stdio).

This moves the existing warning to a new group
(-Wcast-function-type-mismatch), puts the new group under the existing
-Wcast-function-type warning group, and adds
-Wcast-function-type-mismatch to -Wextra.