DeclContext: Fix iterator category

This amends 0cb7e7ca0c864e052bf49978f3bcd667c9e16930.
The iterator category of lookup_iterator was changed, but here it stayed
hardcoded as random access. Found while trying to build Clazy.

Differential-Revision: https://reviews.llvm.org/D100590
GitOrigin-RevId: 46968577336f29ddc5fd30e87cb94b03fa3c9cd9
1 file changed