Make CXXBaseSpecifier::getType return unqual type.

Various pieces of code, like base initialization in Sema and RTTI IRGen,
don't properly ignore qualifiers on base classes.  Instead of auditing the
whole codebase, just strip them off in the getter.  (The type as written is
still available in the TypeSourceInfo for code that cares.)

Fixes PR16596.

llvm-svn: 186125