fix(typescript-estree): correct AST regression introduced by TS4.0 upgrade

[bradzacher]

Reference: prettier/prettier#8805

I had a major brainfart when implementing #2305 and didn't think about the fact that changes should be backwards compatible.

When handling the null type change introduced by 4.0, I deleted the old branches instead of gating them behind a version check. This PR just gates the old logic for < 4.0.

We don't have automated testing against old TS versions yet (#1752) so I tested this by hand by doing the following:

• yarn build to build against 4.0.0-beta.
• change the resolved ts version to 3.9.7
• yarn install --ignore-scripts (the automatic rebuild after the install would fail due to incorrect types)
• cd packages/typescript-estree && yarn test ast-fixtures
• inspected the test output to ensure the only failures were due to fixtures for 4.0 features.

cc @thorn0 / @fisker

[typescript-eslint]

Thanks for the PR, @bradzacher!

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[codecov]

Codecov Report

Merging #2316 into master will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #2316      +/-   ##
==========================================
- Coverage   93.10%   93.09%   -0.02%     
==========================================
  Files         283      284       +1     
  Lines        9081     9092      +11     
  Branches     2493     2495       +2     
==========================================
+ Hits         8455     8464       +9     
- Misses        301      302       +1     
- Partials      325      326       +1     

Flag	Coverage Δ
#unittest	93.09% <80.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/typescript-estree/src/node-utils.ts	96.72% <50.00%> (-0.02%)	⬇️
packages/typescript-estree/src/convert.ts	98.35% <60.00%> (-0.27%)	⬇️
packages/typescript-estree/src/version-check.ts	100.00% <100.00%> (ø)