A few patches to clean up the XDR code for ImmutableScriptData as well as harden XDR coding further again memory/disk corruption.

Harden XDR code against memory / disk corruption by using the codeEnum32
helper instead of small integers. This is only once per CompilationStencil so
size impact is neglible.

This function is only being used in that file now, so move the code and mark it
as static.

Depends on D122153

Transcoding the SharedImmutableScriptData and ImmutableScriptData together gives
more flexibility. A later patch will support handling a nullptr sisd by coding
a length of zero instead.

Depends on D122154

When transcoding a js::frontend::SharedDataContainer that is in "vector"
configuration, we may encounter nullptrs in the vector. Instead of coding a byte
for each entry to determine if it is nullptr or not, start coding the
ImmutableScriptData size with a value of zero. In "vector" mode, this container
is dense and most entries are non-nullptr with a non-zero size anyways. Overall,
this should reduce the size of XDR data needed by removing the is-empty byte and
associated alignment padding.

Depends on D122155

In Bug 1710984 we added extra checks to avoid certain crashes that looked like
memory corruption. This patch makes those checks slightly more robust by
checking the 'optional-offsets' array is in-bounds before dereferencing it for
the rest of the checks.

Depends on D122156

Here is an example of the crash that the last patch helps: https://crash-stats.mozilla.org/report/index/b4dcfd9a-9117-4e8a-bd09-2533e0210809

https://hg.mozilla.org/mozilla-central/rev/f1ad83b1e704
https://hg.mozilla.org/mozilla-central/rev/019d13cbb22f
https://hg.mozilla.org/mozilla-central/rev/58f3627453a7
https://hg.mozilla.org/mozilla-central/rev/5fdc6f93f436
https://hg.mozilla.org/mozilla-central/rev/efefbf74d3fc