Debug Info: Add basic support for external types references.

This is a necessary prerequisite for bootstrapping the emission
of debug info inside modules.

- Adds a FlagExternalTypeRef to DICompositeType.
  External types must have a unique identifier.
- External type references are emitted using a forward declaration
  with a DW_AT_signature([DW_FORM_ref_sig8]) based on the UID.

http://reviews.llvm.org/D9612

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242302 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/DebugInfoMetadata.h

@@ -574,6 +574,7 @@ public:
   bool isStaticMember() const { return getFlags() & FlagStaticMember; }
   bool isLValueReference() const { return getFlags() & FlagLValueReference; }
   bool isRValueReference() const { return getFlags() & FlagRValueReference; }
+  bool isExternalTypeRef() const { return getFlags() & FlagExternalTypeRef; }
 
   DITypeRef getRef() const { return DITypeRef::get(this); }