[clang-tidy] Harden PreferMemberInitializerCheck

Prevent warning when the values are initialized using fields that will be initialized later or VarDecls defined in the constructors body.
Both of these cases can't be safely fixed.
Also improve logic of finding where to insert member initializers, previously it could be confused by in class member initializers.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D97132