make common_type SFINAE-friendly and support void. Patch from Agustin Berge.

This patch also fixes PR22135. (https://llvm.org/bugs/show_bug.cgi?id=22135)
See the review for more information: http://reviews.llvm.org/D6964


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@246977 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed