CodeGen: Move the call to DwarfDebug::beginModule() out of the constructor.

[pcc]

Summary

This gives AsmPrinter a chance to initialize its DD field before
we call beginModule(), which is about to start using it.

[aprantl]

I wanted to suggest a change in the doxygen comment to DwarfDebug::beginModule() to indicate that this is now supposed to be called from the outside, but it looks like the existing comment describes the new usage pattern better than the current one :-)
LGTM.

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

if (MAI->doesSupportDebugInformation()) {
     bool EmitCodeView = MMI->getModule()->getCodeViewFlag();
     if (EmitCodeView && TM.getTargetTriple().isKnownWindowsMSVCEnvironment()) {
       Handlers.push_back(HandlerInfo(new CodeViewDebug(this),
                                      DbgTimerName,
                                      CodeViewLineTablesGroupName));
     }
     if (!EmitCodeView || MMI->getModule()->getDwarfVersion()) {
       DD = new DwarfDebug(this, &M);
+      DD->beginModule();
       Handlers.push_back(HandlerInfo(DD, DbgTimerName, DWARFGroupName));
     }
   }
 
   EHStreamer *ES = nullptr;
   switch (MAI->getExceptionHandlingType()) {
   case ExceptionHandling::None:
     break;

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

DwarfDebug::DwarfDebug(AsmPrinter *A, Module *M)
   // See GDB bug 11616 - DW_OP_form_tls_address is unimplemented
   // https://sourceware.org/bugzilla/show_bug.cgi?id=11616
   UseGNUTLSOpcode = tuneForGDB() || DwarfVersion < 3;
 
   // GDB does not fully support the DWARF 4 representation for bitfields.
   UseDWARF2Bitfields = (DwarfVersion < 4) || tuneForGDB();
 
   Asm->OutStreamer->getContext().setDwarfVersion(DwarfVersion);
-
-  {
-    NamedRegionTimer T(DbgTimerName, DWARFGroupName, TimePassesIsEnabled);
-    beginModule();
-  }
 }
 
 // Define out of line so we don't have to include DwarfUnit.h in DwarfDebug.h.
 DwarfDebug::~DwarfDebug() { }
 
 static bool isObjCClass(StringRef Name) {
   return Name.startswith("+") || Name.startswith("-");
 }
void DwarfDebug::constructAndAddImportedEntityDIE(DwarfCompileUnit &TheCU,
   if (DIE *D = TheCU.getOrCreateContextDIE(N->getScope()))
     D->addChild(TheCU.constructImportedEntityDIE(N));
 }
 
 // Emit all Dwarf sections that should come prior to the content. Create
 // global DIEs and emit initial debug info sections. This is invoked by
 // the target AsmPrinter.
 void DwarfDebug::beginModule() {
+  NamedRegionTimer T(DbgTimerName, DWARFGroupName, TimePassesIsEnabled);
   if (DisableDebugInfoPrinting)
     return;
 
   const Module *M = MMI->getModule();
 
   unsigned NumDebugCUs = 0;
   for (DICompileUnit *CUNode : M->debug_compile_units()) {
     (void)CUNode;