Increase the size of PMTopLevelManager::ImmutablePasses from 8 to 16.

During a bootstrap build of Clang, this SmallVector always held 14 elements.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215333 91177308-0d34-0410-b5e6-96231b3b80d8