In C++14 onwards, it is permitted to read mutable members in constant

In C++14 onwards, it is permitted to read mutable members in constant
expressions, if their lifetime began during the evaluation of the expression.

This is technically not allowed in C++11, though we could consider permitting
it there too, as an extension.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@325663 91177308-0d34-0410-b5e6-96231b3b80d8