[libc++] Explicitly cast in generate_canonical

A new clang warning introduced in r367497 was complaining about
the change in value.

Thanks to Brian Cain for the patch.
Differential Revision: https://reviews.llvm.org/D66422

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369393 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed