[ADT] Fix definition of `adl_begin`/`adl_end` and `Iter`/`ValueOfRange`

- Make `IterOfRange` and `ValueOfRange` work with types that require
  custom `begin`/`end` functions.
- Allow for `adl_begin`/`adl_end` to be used in constant-evaluated
  contexts.
- Use SFINAE-friendly trailing return type deductions `adl_begin`/`adl_end` so that they are useable in template argument deduction.
- Add missing documentation comments.

This is required for future work in https://reviews.llvm.org/D144503.

Reviewed By: dblaikie, zero9178

Differential Revision: https://reviews.llvm.org/D144583