[C++][Sema] Ignore top-level qualifiers in casts
Ignore top-level qualifiers in casts, which fixes issues in reinterpret_cast.
This rule comes from [expr.type]/8.2.2 which explains that casting to a
pr-qualified type should actually cast to the unqualified type. In C++
this is only done for types that aren't classes or arrays.
Reviewed By: Anastasia
Differential Revision: https://reviews.llvm.org/D102689