Suppress undefined-template warnings when the pattern is declared in a system header.

The way to fix an undefined-template warning is to add lines to the header file that defines the template pattern. We should suppress the warnings when the template pattern is in a system header because we don't expect users to edit those.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321665 91177308-0d34-0410-b5e6-96231b3b80d8