[clang][dataflow] Add captured parameters to ReferencedDecls for lamb… (#117771)

…da call operators.

This doesn't require that they be used in the operator's body, unlike
other ReferencedDecls. This is most obviously different from captured
local variables, which can be captured but will not appear in
ReferencedDecls unless they appear in the operator's body.

This difference simplifies the collection of the captured parameters,
but probably could be eliminated if desirable.