[Version 10.0] Feature support for assignment and declaration in same deconstruction

[RexJaeschke]

There is no corresponding MS proposal.

Prior to V10, the tuple elements in a deconstructing assignment all had to be declaration_expressions that designated new local variables, possibly with discards, OR they all had to be identifiers designating existing variables, possibly with discards, but not a combination of old and new variable designators.

V10 allows a combination, as in

int z;
(int x, _, z) = (2, 4, 6);

This is achieved simply by removing the restriction on allowing such a combination.

[Nigel-Ecma]

The changes required for this are covered by #1538 (in review at the time or writing). That PR is for v8 but contains the changes required for v10 in HTML comments tagged with [TODO]. If #1538 is merged this issue becomes “apply the HTML comments for v10”.