[openmp][nfc] Replace OMPGridValues array with struct

[nfc] Replaces enum indices into an array with a struct. Named the
fields to match the enum, leaves memory layout and initialization unchanged.

Motivation is to later safely remove dead fields and replace redundant ones
with (compile time) computation. It should also be possible to factor some
common fields into a base and introduce a gfx10 amdgpu instance with less
duplication than the arrays of integers require.

Reviewed By: ronlieb

Differential Revision: https://reviews.llvm.org/D108339