Add 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. All this does at the moment is set a flag
codegenopts.

http://reviews.llvm.org/D11958

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246192 91177308-0d34-0410-b5e6-96231b3b80d8