The 'constexpr implies const' rule for non-static member functions is gone in

The 'constexpr implies const' rule for non-static member functions is gone in
C++1y, so stop adding the 'const' there. Provide a compatibility warning for
code relying on this in C++11, with a fix-it hint. Update our lazily-written
tests to add the const, except for those ones which were testing our
implementation of this rule.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179969 91177308-0d34-0410-b5e6-96231b3b80d8