[PM] Add support for parsing function passes and function pass manager
nests to the opt commandline support. This also showcases the
implicit-initial-manager support which will be most useful for testing.
There are several bugs that I spotted by inspection here that I'll fix
with test cases in subsequent commits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199038 91177308-0d34-0410-b5e6-96231b3b80d8