Trust Kotlin compiler to identify unsafe casts

[3flex]

UnsafeCast now trusts the Kotlin compiler to identify casts that won't succeed. This should eliminate false positives though the detection rate for actual issues will drop, as some "safe" casts that the compiler cannot always tell will fail may still cause problems.

Now requires type/symbol resolution enabled in the detekt run for the rule to work.

Fixes #1601
Fixes #1137 (I couldn't replicate the original issue by just removing the suppression lines in ThresholdedCodeSmellAssert and rerunning CI in AppVeyor, though this should resolve it anyway).

[schalkms]

Great! This PR contains some nice improvements and refactorings.

[3flex]

Blocked by #1880

[codecov-io]

Codecov Report

Merging #1879 into master will decrease coverage by <.01%.
The diff coverage is 51.85%.

@@             Coverage Diff              @@
##             master    #1879      +/-   ##
============================================
- Coverage     80.45%   80.44%   -0.01%     
- Complexity     1968     1969       +1     
============================================
  Files           326      326              
  Lines          5510     5513       +3     
  Branches       1022     1019       -3     
============================================
+ Hits           4433     4435       +2     
  Misses          543      543              
- Partials        534      535       +1

Impacted Files	Coverage Δ	Complexity Δ
...turbosch/detekt/rules/KtClassOrObjectExtensions.kt	66.66% <ø> (-8.34%)	0 <0> (ø)
...gitlab/arturbosch/detekt/rules/style/MayBeConst.kt	75% <0%> (-2.28%)	40 <2> (-1)
...rturbosch/detekt/rules/style/UnusedPrivateClass.kt	67.85% <0%> (ø)	4 <0> (ø)	⬇️
...bosch/detekt/rules/bugs/EqualsWithHashCodeExist.kt	89.47% <0%> (+3.75%)	9 <3> (ø)	⬇️
...turbosch/detekt/rules/style/UnusedPrivateMember.kt	93.47% <100%> (-0.08%)	5 <0> (ø)
...itlab/arturbosch/detekt/rules/style/ReturnCount.kt	87.75% <100%> (+2.04%)	20 <0> (ø)	⬇️
...etekt/rules/style/FunctionOnlyReturningConstant.kt	92% <100%> (-0.31%)	27 <0> (-1)
...sch/detekt/rules/naming/MatchingDeclarationName.kt	92.85% <100%> (+7.14%)	7 <0> (ø)	⬇️
.../gitlab/arturbosch/detekt/rules/bugs/UnsafeCast.kt	75% <40%> (-13.89%)	4 <1> (-1)
...sch/detekt/rules/style/UnnecessaryAbstractClass.kt	91.17% <50%> (+2.28%)	15 <0> (ø)	⬇️
... and 5 more

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