P0962R1: only use the member form of 'begin' and 'end' in a range-based

P0962R1: only use the member form of 'begin' and 'end' in a range-based
for loop if both members exist.

This resolves a DR whereby an errant 'begin' or 'end' member in a base
class could result in a derived class not being usable as a range with
non-member 'begin' and 'end'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@342925 91177308-0d34-0410-b5e6-96231b3b80d8