Disallow complex expressions on left-hand-sides of assignments.

https://github.com/ethereum/solidity/pull/13139 attempted to add some warnings about easy-to-miss dangling reference cases.

We could go beyond that and also warn about cases like this:
```
contract C {
uint[] x;
uint[] y;
function g() internal returns (uint[] storage) {
x.pop();
return y;
}
function f() public {
(x[x.length - 1], g()[1]) = (1, 2);
}
}
```
(which, roughly, we got reported as a bug bounty report)

In general we could warn about using most kinds of complex expressions on left-hand-sides of tuple assignments, especially about function calls.

We should also check if we can construct problematic cases in non-tuple assignments.

Review pull request #13139 and the tuple-assignment example in this issue first. Investigate how complex left-hand expressions are handled in tuple and non-tuple assignments, and consider the work complete when problematic cases are consistently diagnosed without missing the reported dangling-reference behavior.

Written by the indexing model from the issue text.