[ThinLTO] Fix dot dumper for full-LTO modules

[tejohnson]

LGTM with one fix:
I guess this is because the module id for the regular lto module is -1? Please add that to the patch summary before committing.

llvm/trunk/lib/IR/ModuleSummaryIndex.cpp

void ModuleSummaryIndex::exportToDot(raw_ostream& OS) const {
   // Get node identifier in form MXXX_<GUID>. The MXXX prefix is required,
   // because we may have multiple linkonce functions summaries.
   auto NodeId = [](uint64_t ModId, GlobalValue::GUID Id) {
     return ModId == (uint64_t)-1 ? std::to_string(Id)
                                  : std::string("M") + std::to_string(ModId) +
                                        "_" + std::to_string(Id);
   };
 
-  auto DrawEdge = [&](const char *Pfx, int SrcMod, GlobalValue::GUID SrcId,
+  auto DrawEdge = [&](const char *Pfx, uint64_t SrcMod, GlobalValue::GUID SrcId,
                       int DstMod, GlobalValue::GUID DstId, int TypeOrHotness) {
     // 0 corresponds to alias edge, 1 to ref edge, 2 to call with unknown
     // hotness, ...
     TypeOrHotness += 2;
     static const char *EdgeAttrs[] = {
         " [style=dotted]; // alias",
         " [style=dashed]; // ref",
         " // call (hotness : Unknown)",

llvm/trunk/test/ThinLTO/X86/dot-dumper-full-lto.ll

+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: opt -module-summary %p/Inputs/dot-dumper.ll -o %t2.bc
+; RUN: llvm-lto2 run -save-temps %t1.bc %t2.bc -o %t3 \
+; RUN:  -r=%t1.bc,main,px \
+; RUN:  -r=%t1.bc,A, \
+; RUN:  -r=%t2.bc,foo,p \
+; RUN:  -r=%t2.bc,bar,p \
+; RUN:  -r=%t2.bc,A,p \
+; RUN:  -r=%t2.bc,B,p
+; RUN: cat %t3.index.dot | FileCheck %s
+
+; CHECK: subgraph cluster_4294967295
+; CHECK:   M4294967295_[[ID:[0-9]+]]{{.*}}main
+; CHECK: // Cross-module edges:
+; CHECK:  M4294967295_[[ID]] -> M0_{{[0-9]+}}{{.*}}// ref
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@A = external global i32
+
+define i32 @main() {
+  %v = load i32, i32* @A
+  ret i32 %v
+}
+
+!0 = !{i32 1, !"ThinLTO", i32 0}
+!llvm.module.flags = !{ !0 }