[mlir][OpenMP] map argument to reduction initialization region

The argument to the initialization region of reduction declarations was
never mapped. This meant that if this argument was accessed inside the
initialization region, that mlir operation would be translated to an
llvm operation with a null argument (failing verification).

Adding the mapping ensures that the right LLVM value can be found when
inlining and converting the initialization region.

We have to separately establish and clean up these mappings for each use
of the reduction declaration because repeated usage of the same
declaration will inline it using a different concrete value for the
block argument.