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/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -467,7 +467,10 @@ void DwarfDebug::beginModule() {
     for (auto *Ty : CUNode->getRetainedTypes()) {
       // The retained types array by design contains pointers to
       // MDNodes rather than DIRefs. Unique them here.
-      CU.getOrCreateTypeDIE(cast<DIType>(resolve(Ty->getRef())));
+      DIType *RT = cast<DIType>(resolve(Ty->getRef()));
+      if (!RT->isExternalTypeRef())
+        // There is no point in force-emitting a forward declaration.
+        CU.getOrCreateTypeDIE(RT);
     }
     // Emit imported_modules last so that the relevant context is already
     // available.
@@ -1884,7 +1887,7 @@ MCDwarfDwoLineTable *DwarfDebug::getDwoLineTable(const DwarfCompileUnit &CU) {
   return &SplitTypeUnitFileTable;
 }
 
-static uint64_t makeTypeSignature(StringRef Identifier) {
+uint64_t DwarfDebug::makeTypeSignature(StringRef Identifier) {
   MD5 Hash;
   Hash.update(Identifier);
   // ... take the least significant 8 bytes and return those. Our MD5