[openacc] Add attribute to hold declare data clause information

[openacc] Add attribute to hold declare data clause information

For variables in declare clauses, their producing operation should be
marked with the data clause for ease of lookup and consistency
verification. Thus add an attribute that can be used for this purpose
plus verification that declare data operation matches the declare
data clause on variable.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D155640