fix(eslint-plugin): [unbound-method] false positives for unary expressions

[auhsoja]

Fixes #1934.

Added an extra check to isSafeUse to deal with the case when an unbound method is made truthy/falsy (and therefore safe to reference) through the use of the ! operator.

[typescript-eslint]

Thanks for the PR, @auhsoja!

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 #1964 into master will decrease coverage by 0.49%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1964      +/-   ##
==========================================
- Coverage   94.38%   93.88%   -0.50%     
==========================================
  Files         164      170       +6     
  Lines        7585     7737     +152     
  Branches     2180     2227      +47     
==========================================
+ Hits         7159     7264     +105     
- Misses        183      218      +35     
- Partials      243      255      +12     

Flag	Coverage Δ
#unittest	93.88% <100.00%> (-0.50%)	⬇️

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/unbound-method.ts	95.23% <100.00%> (ø)
...nt-plugin/src/rules/switch-exhaustiveness-check.ts	97.67% <0.00%> (-2.33%)	⬇️
...kages/eslint-plugin/src/rules/no-base-to-string.ts	97.95% <0.00%> (-2.05%)	⬇️
...ackages/eslint-plugin/src/rules/member-ordering.ts	98.46% <0.00%> (-0.69%)	⬇️
packages/eslint-plugin/src/util/types.ts	81.21% <0.00%> (-0.39%)	⬇️
...ackages/eslint-plugin/src/rules/prefer-includes.ts	100.00% <0.00%> (ø)
...kages/experimental-utils/src/eslint-utils/index.ts	100.00% <0.00%> (ø)
...es/eslint-plugin/src/rules/no-floating-promises.ts	100.00% <0.00%> (ø)
...xperimental-utils/src/eslint-utils/applyDefault.ts	100.00% <0.00%> (ø)
...-plugin/src/rules/restrict-template-expressions.ts	100.00% <0.00%> (ø)
... and 8 more

[bradzacher]

LGTM - thanks for doing this!