[Docs] CodingStandards: for_each is discouraged

Summary:
As per disscussion in D83351, using `for_each` is potentially confusing,
at least in regards to inconsistent style (there's less than 100 `for_each`
usages in LLVM, but ~100.000 `for` range-based loops

Therefore, it should be avoided.

Reviewers: dblaikie, nickdesaulniers

Reviewed By: dblaikie, nickdesaulniers

Subscribers: hubert.reinterpretcast, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83431

llvm/docs/CodingStandards.rst

@@ -1302,6 +1302,9 @@ loops wherever possible for all newly added code. For example:
   for (Instruction &I : *BB)
     ... use I ...
 
+Usage of ``std::for_each()``/``llvm::for_each()`` functions is discouraged,
+unless the the callable object already exists.
+
 Don't evaluate ``end()`` every time through a loop
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^