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. All this does at the moment is set a flag
codegenopts.
http://reviews.llvm.org/D11958
llvm-svn: 246192