[docs] Add section on iteration utilities (`zip` & `enumerate`)

[kuhar]

Summary

Since these are not in C++17, mention them in the programmers manual.

[kuhar]

Add missing backticks

[kazu]

Thank you for documenting these!

llvm/docs/ProgrammersManual.rst

 for a bit in a CoalescingBitVector is O(log(gaps between contiguous ranges)).
 
 CoalescingBitVector is a better choice than BitVector when gaps between ranges
 of set bits are large. It's a better choice than SparseBitVector when find()
 operations must have fast, predictable performance. However, it's not a good
 choice for representing sets which have lots of very short ranges. E.g. the set
 `{2*x : x \in [0, n)}` would be a pathological input.
 
+.. _utility_functions:
+
+Useful Utility Functions
+========================
+
+LLVM implements a number of general utility functions used acrossed the
+codebase. You can find the most common ones in ``STLExtras.h``
+(`doxygen <https://llvm.org/doxygen/STLExtras_8h.html>`__). Some of these wrap
+well-known C++ standard library functions, while others are unique to LLVM.
+
+.. _uf_iteration:
+
+Iterating over ranges
+---------------------
+
+Sometimes you may want to iterate over more than range at a time or know the
+index of the index. LLVM provides custom utility functions to make that easier,
+without having to manually manage all iterators and/or indices:
+
+.. _uf_zip:
+
+The ``zip``\ * functions
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+``zip``\ * functions allow for iterating over elements from two or more ranges
+at the same time. For example:
+
+.. code-block:: c++
+
+    SmallVector<size_t> Counts = ...;
+    char Letters[26] = ...;
+    for (auto [Letter, Count] : zip_equal(Letters, Counts))
+      errs() << Letter << ": " << Count << "\n";
+
+Note that the elements are provided through a 'reference wrapper' proxy type
+(tuple of references), which combined with the structured bindings declaration
+makes ``Letter`` and ``Count`` references to range elements. Any modification
+to these references will affect the elements of ``Letters`` or ``Counts``.
+
+The ``zip``\ * functions support temporary ranges, for example:
+
+.. code-block:: c++
+
+    for (auto [Letter, Count] : zip(SmallVector<char>{'a', 'b', 'c'}, Counts))
+      errs() << Letter << ": " << Count << "\n";
+
+The difference between the functions in the ``zip`` family is how they behave
+when the supplied ranges have different lengths:
+
+* ``zip_equal`` -- requires all input ranges have the same length.
+* ``zip`` -- iteration stops when the end of the shortest range is reached.
+* ``zip_first`` -- requires the first range is the shortest one.
+* ``zip_longest`` -- iteration continues until the end of the longest range is
+  reached. The non-existent elements of shorter ranges are replaced with
+  ``std::nullopt``.
+
+The length requirements are checked with ``assert``\ s.
+
+As a rule of thumb, prefer to use ``zip_equal`` when you expect all
+ranges to have the same lengths, and consider alternative ``zip`` functions only
+when this is not the case. This is because ``zip_equal`` clearly communicates
+this same-length assumption and has the best (release-mode) runtime performance.
+
+.. _uf_enumerate:
+
+``enumerate``
+^^^^^^^^^^^^^
+
+The ``enumerate`` functions allows to iterate over one or more ranges while
+keeping track of the index of the current loop iteration. For example:
+
+.. code-block:: c++
+
+    for (auto [Idx, BB, Value] : enumerate(Phi->blocks(),
+                                           Phi->incoming_values()))
+      errs() << "#" << Idx << " " << BB->getName() << ": " << *Value << "\n";
+
+The current element index is provided as the first structured bindings element.
+Alternatively, the index and the element value can be obtained with the
+``index()`` and ``value()`` member functions:
+
+.. code-block:: c++
+
+    char Letters[26] = ...;
+    for (auto En : enumerate(Letters))
+      errs() << "#" << En.index() << " " << En.value() << "\n";
+
+Note that ``enumerate`` has ``zip_equal`` semantics and provides elements
+through a 'reference wrapper' proxy, which makes them modifiable when accessed
+through structured bindings or the ``value()`` member function. When two or more
+ranges are passed, ``enumerate`` requires them to have equal lengths (checked
+with an ``assert``).
+
 .. _debugging:
 
 Debugging
 =========
 
 A handful of `GDB pretty printers
 <https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing.html>`__ are
 provided for some of the core LLVM libraries. To use them, execute the