Increase SmallVector<> size in DiagnosticsEngine::setSeverityForGroup

In a Clang bootstrap, this vector always held 129 elements.

Also switch to a range-based for loop.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215360 91177308-0d34-0410-b5e6-96231b3b80d8