(Resubmitted with Phabricator for easier reviewing.)
This fixes an assertion failure in assignment if an lvalue is constant but its
type is not const-qualified.
An expression may be MLV_ConstQualified either because the type is actually
“const” or because it’s in the OpenCL constant address space (see
ExprClassification.cpp:608). Unfortunately, isReferenceToConstCapture asserts
that the type must actually be “const”, which is not necessarily true. This
patch changes the assertion to what I think is intended.