[analyzer] Deprecate FAM analyzer-config, recommend -fstrict-flex-arrays instead

By default, clang assumes that all trailing array objects could be a
FAM. So, an array of undefined size, size 0, size 1, or even size 42 is
considered as FAMs for optimizations at least.

One needs to override the default behavior by supplying the
`-fstrict-flex-arrays=<N>` flag, with `N > 0` value to reduce the set of
FAM candidates. Value `3` is the most restrictive and `0` is the most
permissive on this scale.

0: all trailing arrays are FAMs
1: only incomplete, zero and one-element arrays are FAMs
2: only incomplete, zero-element arrays are FAMs
3: only incomplete arrays are FAMs

If the user is happy with consdering single-element arrays as FAMs, they
just need to remove the
`consider-single-element-arrays-as-flexible-array-members` from the
command line.
Otherwise, if they don't want to recognize such cases as FAMs, they
should specify `-fstrict-flex-arrays` anyway, which will be picked up by
CSA.

Any use of the deprecated analyzer-config value will trigger a warning
explaining what to use instead.
The `-analyzer-config-help` is updated accordingly.

Depends on D138657

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D138659