We treated DependentSizedArrayTypes with the same element type but
differing size expressions as equivalently canonical.  This would lead
to bizarre behavior during template instantiation.

This fixes PR24212.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243093 91177308-0d34-0410-b5e6-96231b3b80d8