Replace "failed template argument deduction" diagnostic with something useful
in the one case where we've already factored out a reason code.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@174036 91177308-0d34-0410-b5e6-96231b3b80d8
Allow the computation of the base priority for a declaration code completion result to consider the completion context

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@174037 91177308-0d34-0410-b5e6-96231b3b80d8