C++1y is now C++14!

Changes diagnostic options, language standard options, diagnostic identifiers, diagnostic wording to use c++14 instead of c++1y. It also modifies related test cases to use the updated diagnostic wording.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215982 91177308-0d34-0410-b5e6-96231b3b80d8