[clangd] Return Dex Iterators

The original Dex Iterators patch (https://reviews.llvm.org/rL338017)
caused problems for Clang 3.6 and Clang 3.7 due to the compiler bug
which prevented inferring template parameter (`Size`) in create(And|Or)?
functions. It was reverted in https://reviews.llvm.org/rL338054.

In this revision the mentioned helper functions were replaced with
variadic templated versions.

Proposed changes were tested on multiple compiler versions, including
Clang 3.6 which originally caused the failure.

llvm-svn: 338116