[Flang][Unit Test] Move the declaration of kindMap to the class

kindMap variable is declared in the Setup function but passed as
a reference to the firBuilder class. The firBuilder is declared in
the class and hence its lifetime exceeds that of kindMap. This can
lead to undefined behaviour. Move the kindMap variable into the class
to avoid this.

This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D115631