Reject a slightly-sneaky way to perform a read of mutable state from within a
constexpr function. Part of this fix is a tentative fix for an as-yet-unfiled
core issue (we're missing a prohibition against reading mutable members from
unions via a trivial constructor/assignment, since that doesn't perform an
lvalue-to-rvalue conversion on the members).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217852 91177308-0d34-0410-b5e6-96231b3b80d8