[AST] Use an explicit copy in a range-based for

[AST] Use an explicit copy in a range-based for

The AssociationIteratorTy type will be copied in a range-based for loop.
Make the copy explicit to avoid the -Wrange-loop-analysis warning.

This avoids new warnings due to D68912 adds -Wrange-loop-analysis to -Wall.

Differential Revision: https://reviews.llvm.org/D70045