[flang] Add ExternalNameConversionPass to pass pipeline

This seems to be the consensus in
https://github.com/flang-compiler/f18-llvm-project/issues/1316

The patch adds ExternalNameConversion to the default FIR CodeGen pass
pipeline, right before the FIRtoLLVM pass. It also adds a flag to
optionally disable it, and sets it in `tco`. In other words, `flang-new`
and `flang-new -fc1` will both run the pass by default, whereas `tco`
will not, so none of the tests need to be updated.

Differential Revision: https://reviews.llvm.org/D121171