assert() is compiled out for release builds, but we don't want to
continue running in these impossible situations.
This also resolves the "implicit fallthrough" warnings for the asserts
in switch cases.
Note: there are three changes to ascii_isident. Reverting first two (in
find_special_key and first in get_special_key_code) normally fails the new test
with empty &isident, but reverting the third does not. Hence adding `>` to
&isident.
Ref vim/vim#2389.
First stage: something compiling without klee, but with a buch of dirty
hacks - done.
Second stage: something running under klee, able to emit useful results,
but still using dirty hacks - done.
Third stage: make CMake care about clang argumnets - not done, may be
omitted if proves to be too hard. Not that klee can be run on CI in any
case.
