Mark the methods in the Mangler const.

A const ObjectFile needs to be able to provide its name. For an IRObjectFile,
that means being able to call the mangler. Since each IRObjectFile can have
a different mangling, it is natural for them to contain a Mangler which is
therefore also const.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201113 91177308-0d34-0410-b5e6-96231b3b80d8