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

The std::pair<const clang::ValueDecl *, llvm::ArrayRef<clang::OMPClauseMappableExprCommon::MappableComponent>>
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/D70046