This patch adds a -gmodules option to the driver and a -dwarf-ext-refs to cc1 to enable the use of external type references in the debug info (a.k.a. module debugging).
The driver expands -gmodules to "-g -fmodule-format=obj -dwarf-ext-refs" and passes that to cc1.
Most options that start with -g (e.g., -gdwarf-2) also turn on -g, and module requires object-container-wrapped modules, "-dwarf-ext-refs" been the actual low-level option for turning on external type references.
Rationale for the choice of names (and this is really all there is to review in this patch):
"-gmodules": is meant to pair nicely with "-fmodules"
"-dwarf-ext-refs": Fits into the naming scheme of similar options like "-dwarf-column-info" and "-dwarf-debug-flags". Spelling out the option "-dwarf-external-type-references" seemed to be overkill.
All this does at the moment is set a flag codegenopts. Having this flag in place is a prerequisite for emitting debug info into modules: The debug info for a module needs to use external type references for types defined in (other) modules or we would violate the minimal deserialization requirements (cf. test/PCH/check-deserializations.cpp).