fix(eslint-plugin): [no-unsafe-return] error with <TS3.7

[bradzacher]

Fixes #1807

tested by installing TS@3.6.2 and running the tests.
They passed.

[typescript-eslint]

Thanks for the PR, @bradzacher!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #1815 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1815      +/-   ##
==========================================
- Coverage   94.99%   94.96%   -0.03%     
==========================================
  Files         159      159              
  Lines        7112     7114       +2     
  Branches     2033     2035       +2     
==========================================
  Hits         6756     6756              
- Misses        154      155       +1     
- Partials      202      203       +1

Flag	Coverage Δ
#unittest	94.96% <0%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
packages/eslint-plugin/src/util/types.ts	80.26% <0%> (-1.08%)	⬇️