[ThinLTO] Handle -fdebug-pass-manager for backend invocations via clang

[tejohnson]

Summary

The LTO Config field wasn't being set when invoking a ThinLTO backend
via clang (i.e. for distributed builds).

[danielcdh]

Thanks for the fix.

lib/CodeGen/BackendUtil.cpp

static void runThinLTOBackend(ModuleSummaryIndex *CombinedIndex, Module *M,
   Conf.CPU = TOpts.CPU;
   Conf.CodeModel = getCodeModel(CGOpts);
   Conf.MAttrs = TOpts.Features;
   Conf.RelocModel = getRelocModel(CGOpts);
   Conf.CGOptLevel = getCGOptLevel(CGOpts);
   initTargetOptions(Conf.Options, CGOpts, TOpts, LOpts, HeaderOpts);
   Conf.SampleProfile = std::move(SampleProfile);
   Conf.UseNewPM = CGOpts.ExperimentalNewPassManager;
+  Conf.DebugPassManager = CGOpts.DebugPassManager;
   switch (Action) {
   case Backend_EmitNothing:
     Conf.PreCodeGenModuleHook = [](size_t Task, const Module &Mod) {
       return false;
     };
     break;
   case Backend_EmitLL:
     Conf.PreCodeGenModuleHook = [&](size_t Task, const Module &Mod) {

test/CodeGen/thinlto-debug-pm.c

+// Test to ensure -fdebug-pass-manager works when invoking the
+// ThinLTO backend path with the new PM.
+// RUN: %clang -O2 %s -flto=thin -c -o %t.o
+// RUN: llvm-lto -thinlto -o %t %t.o
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-obj -O2 -o %t2.o -x ir %t.o -fthinlto-index=%t.thinlto.bc -fdebug-pass-manager -fexperimental-new-pass-manager 2>&1 | FileCheck %s
+// CHECK: Running pass:
+
+void foo() {
+}