[lldb][NFC] Remove misleading ModulePass base class for IRForTarget

IRForTarget is never used by a pass manager or any other interface that requires
this class to inherit from `Pass`.

Also IRForTarget doesn't implement the current interface correctly because it
uses the `runOnModule` return value to indicate success/failure instead of
changed/not-changed, so if this ever ends up being used as a pass it would most
likely not work as intended.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D102677