[NFC] Generalize ArraySections to work for OpenACC in the future (#89639)

OpenACC is going to need an array sections implementation that is a
simpler version/more restrictive version of the OpenMP version. 

This patch moves `OMPArraySectionExpr` to `Expr.h` and renames it `ArraySectionExpr`,
 then adds an enum to choose between the two.

This also fixes a couple of 'drive-by' issues that I discovered on the way,
but leaves the OpenACC Sema parts reasonably unimplemented (no semantic
analysis implementation), as that will be a followup patch.