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

Recommit of r317951 and r317951 along with what I believe should fix
the remaining buildbot failures - the target triple should be specified
for both the ThinLTO pre-thinlink compile and backend (post-thinlink)
compile to ensure it is consistent.

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

llvm-svn: 318042

clang/lib/CodeGen/BackendUtil.cpp

@@ -1073,6 +1073,7 @@ static void runThinLTOBackend(ModuleSummaryIndex *CombinedIndex, Module *M,
   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) {

clang/test/CodeGen/thinlto-debug-pm.c

@@ -0,0 +1,10 @@
+// Test to ensure -fdebug-pass-manager works when invoking the
+// ThinLTO backend path with the new PM.
+// REQUIRES: x86-registered-target
+// RUN: %clang_cc1 -o %t.o -flto=thin -fexperimental-new-pass-manager -triple x86_64-unknown-linux-gnu -emit-llvm-bc %s
+// 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() {
+}