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

lib/IR/DIBuilder.cpp

@@ -435,6 +435,18 @@ DISubroutineType *DIBuilder::createSubroutineType(DIFile *File,
   return DISubroutineType::get(VMContext, Flags, ParameterTypes);
 }
 
+DICompositeType *DIBuilder::createExternalTypeRef(unsigned Tag, DIFile *File,
+                                                  StringRef UniqueIdentifier) {
+  assert(!UniqueIdentifier.empty() && "external type ref without uid");
+  auto *CTy =
+      DICompositeType::get(VMContext, Tag, "", nullptr, 0, nullptr, nullptr, 0,
+                           0, 0, DINode::FlagExternalTypeRef, nullptr, 0,
+                           nullptr, nullptr, UniqueIdentifier);
+  // Types with unique IDs need to be in the type map.
+  retainType(CTy);
+  return CTy;
+}
+
 DICompositeType *DIBuilder::createEnumerationType(
     DIScope *Scope, StringRef Name, DIFile *File, unsigned LineNumber,
     uint64_t SizeInBits, uint64_t AlignInBits, DINodeArray Elements,