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

llvm-svn: 246192

clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp

@@ -64,6 +64,7 @@ public:
     // ThreadModel, but the backend expects them to be nonempty.
     CodeGenOpts.CodeModel = "default";
     CodeGenOpts.ThreadModel = "single";
+    CodeGenOpts.DebugTypeExtRefs = true;
     CodeGenOpts.setDebugInfo(CodeGenOptions::FullDebugInfo);
     CodeGenOpts.SplitDwarfFile = OutputFileName;
   }