[clang-tidy] New check readability-prefer-member-initializer

Finds member initializations in the constructor body which can
be placed to the member initializers of the constructor instead.
This does not only improves the readability of the code but also
affects positively its performance. Class-member assignments
inside a control statement or following the first control
statement are ignored.

Differential Revision: https://reviews.llvm.org/D71199