When marking virtual functions as used for a class' vtable, mark all functions

When marking virtual functions as used for a class' vtable, mark all functions
which will appear in the vtable as used, not just those ones which were
declared within the class itself. Fixes an issue reported as comment#3 in
PR12763 -- we sometimes assert in codegen if we try to emit a reference to a
function declaration which we've not marked as referenced. This also matches
gcc's observed behavior.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159895 91177308-0d34-0410-b5e6-96231b3b80d8