Have IRBuilder take a template argument on whether or not to preserve

Have IRBuilder take a template argument on whether or not to preserve
names. This can save a lot of allocations if you aren't going to be
looking at the output.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54546 91177308-0d34-0410-b5e6-96231b3b80d8