Support creating flexible types in tests

Flexible types can not be created in normal user code, but if there's a special classifier `kotlin.internal.flexible.ft` with two type parameters,
its occurrences are replaced by flexible types, e.g. `ft<Int, Int?>` becomes `Int!`
Removing the static import that caused exception on compilation
CHECK_TYPES injected through imports
DescriptorRenderer can now render flexible types for code (i.e. `ft<L, U>`)
toString() for easier debugging
Clean up declarations of runReadAction() etc
KT-6059 Regression: irrelevant compiler error on trait inheritance (worked in M8)

 #KT-6059 Fixed
KT-6077 Compiler does not issue error on unsafe access to ArrayList with nullable values

  #KT-6077 Fixed
Compilation errors fixed
Generation of ++ and += fixed for platform types