Fix plugin-transform-block-scoping const violations

[overlookmotel]

Fixes #13245.

Q	A
Fixed Issues?	Fixes #13245
Patch: Bug Fix?	Yes
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	Yes
Documentation PR Link	n/a
Any Dependency Changes?	No
License	MIT

Fix for #13245.

This PR fixes:

1. The simple case const c = 1; c = y++;
2. Assignment expressions const c = 1; c += y++
3. Update expressions const c = 1; c++
4. An unrelated bug with for in - see addition to no-for-in/exec.js test for what was previously failing

The tests are maybe written a bit weirdly. My intent was to keep const declarations in same function scope as they were previously, in case that's part of the purpose of the tests. It's a pain testing side effects of code that throws.

I'm leaving the bugs with destructuring for someone else! When/if this PR is merged, I'll open a separate issue for that specifically.

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[nicolo-ribaudo]

Thanks!

[nicolo-ribaudo]

I think we can simplify this to t.unaryExpression("+", violation.get("argument").node) (even if it saves literally a single byte).

[overlookmotel]

Yes, +c has same effect of triggering .valueOf() as c + 1. Have added a commit to make this change.

I hadn't really bothered with optimization since in most cases code like this is a bug anyway.

Have also added another commit for the other most obvious optimization - shorten c = f() to just f() since the assignment will never be executed anyway. There are other optimizations which could be made (a += 'long_string' to a + '' for example), but in my view going further would add complexity for little gain.

[overlookmotel]

Added one more commit to shorten c &&= x++ -> c && x++.

[overlookmotel]

Thanks for merging.