[Sema] Iteratively strip sugar when removing address spaces.

ASTContext::removeAddrSpaceQualType does not properly deal
with sugar. QualTypes derive their ASes from the AS on the
canonical type, not the type itself. However,
removeAddrSpaceQualType only strips the outermost qualifiers,
which means that it can fail to remove addrspace qualifiers
if there is sugar in the way.

Change the function to desugar types until the address space
really no longer exists on the corresponding QualType. This
should guarantee the removal of the address space.

This fixes the erroneous behavior in D62574.

Reviewed By: rjmccall, svenvh

Differential Revision: https://reviews.llvm.org/D83325