Drop items without source text in Code_matcher

[nmatschke]

Code_matcher assumes that the items following an inline block's anchor are
exactly the items that were parsed from the source file, and rewrites their source text
in place. That assumption breaks when the anchor is itself inside code generated by
another ppx. In particular, ppx_template wraps template-mangled definitions in (**/**)
markers so odoc hides them, and those markers inherit the location of the node they were
generated from. So

[%%template:
type t = int [@@kind.explicit k = (value, value_or_null)] [@@deriving_inline globalize]

[@@@end]]

expands to a (**/**) item whose location is the whole type declaration, and the
corrected code replaces the declaration with the derived signature instead of inserting
it after.

Fix this by dropping items that start before the end of the anchor item, since those
cannot correspond to source text following it.

[NathanReb]

The change looks sane to me but I think it is worth a test!

[NathanReb]

Could you also rebase it on top of main to fix the 5.5 builds in the CI, sign-off your commits with -s and add a changelog entry please?

[NathanReb]

This sounds a little bit odd, I think adding the ppx_template example here would make a bit more sense as otherwise, ppx generated deriving_inline nodes sounds a bit crazy and something we don't necessarily want to support.

[NathanReb]

Can you elaborate a bit more on ppx_template and explain what it does and why a ppx would generate deriving_inline nodes?