[OpenACC][NFC] AST changes for Reduction combiner (#162573)

This is the first patch of a handful to get the reduction combiner
recipe lowering properly. THIS patch is NFC as it doesn't actually
change anything except the structure of the AST.

For each 'combiner' recipe we need a 'LHS' 'RHS' and expression to
represent the operation.

Each var-reference can have 1 or more combiners.

IF it is a plain scalar, or a struct with the proper operator, or an
array of either of those, there will be 1.

HOWEVER, aggregates without the proper operator are supposed to be
broken down and done from their elements (which can only be scalars). In
this case, we will represent 1 'combiner' recipe per field-decl.

This patch only puts the infrastructure in place to do so, future
patches wll do the work to fill this in.

0 files changed, 0 insertions, 0 deletions