Problem:    Obvious mistakes are accepted as valid expressions.
Solution:   Be more strict about parsing numbers. (Yasuhiro Matsumoto,
            closes vim/vim#3981)
https://github.com/vim/vim/commit/16e9b85113e0b354ece1cb4f5fcc7866850f3685

Update vim_str2nr_spec.lua to add more tests that use strict = true.

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.

UBSAN with clang 10.0 is checking for adding offsets to a `NULL` pointer
which is not allowed. This is not yet checked in the version of clang
used in CI (7.0.0). I will work on cases of this so that tests passes
locally for me.

This could be tested in CI by either upgrading the clang of the
ASAN/UBSAN to 10.0, or add yet another CI target which builds with
clang 10.0.

vim-patch:7.4.2058

Instead of declaring an enum, this creates a global variable. As gcc10
uses -fno-common by default, global variables declared with the same
name more than once is not allowed anymore revealing this issue.

Each time this header is included, we define the enum name as a global
variable.

See also https://bugzilla.redhat.com/show_bug.cgi?id=1799680

Also revert possibly-wrong change in f_sign_getplaced() from
652be3cb0040.