[win][arm64ec] Handle empty function names (#151609)
While testing Arm64EC, I observed that LLVM crashes when an empty
function name is used. My original fix in #151409 was to raise an error,
but this change now handles the empty name via
`Mangler::getNameWithPrefix` (which assigns a name to the function).
To get this working, I had to create the `Mangler` in
`TargetLoweringObjectFile` early so it would be available to Arm64EC's
lowering. There's no reason why `Mangler` is only created when
`Initialize` is called (or re-created if it exists), and so I moved
creation to the constructor and switched the raw pointer for a
`unique_ptr` to avoid the explicit `delete` in the destructor.
