[Modules] Move the PassNameParser to the IR library as it deals in the
PassInfo structures of the legacy pass manager. Also give it the Legacy
prefix as it is not a particularly widely used header.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202839 91177308-0d34-0410-b5e6-96231b3b80d8