When determining whether we can make a declaration into a global

When determining whether we can make a declaration into a global
constant, also consider whether it's a class type that has any mutable
fields. If so, it can't be a global constant.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131276 91177308-0d34-0410-b5e6-96231b3b80d8