Instantiate fewer templates in PassManager::addPass()

We create many instantiations of PassManager::addPass() in
PassBuilder.cpp.  vector::emplace_back() and make_unique() are both
templated and would have many instantiations based on the number of
times we instantiate addPass().  Now we directly construct the
unique_ptr with the type as the actual unique_ptr type in the vector we
are adding it to, so we only have one unique_ptr constructor
instantiation across all addPass() instantiations and only the
non-templated push_back().

This makes PassBuilder.cpp slightly faster to build.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D110775