[ADT][NFCI] Do not use non-const lvalue-refs with enumerate in llvm/

Replace references to `enumerate` results with either const lvalue
rerences or structured bindings. I did not use structured bindings
everywhere as it wasn't clear to me it would improve readability.

This is in preparation to the switch to `zip` semantics which won't
support non-const lvalue reference to elements:
https://reviews.llvm.org/D144503.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D145987