[ThinLTO] Make sure the Context used for the ThinLTO backend has all the appropriate options

An important performance setting on the LLVMContext for LTO is
enableDebugTypeODRUniquing(), this adds an automatic merging of
debug information in the context based on type ids.

Also, the lto::Config includes a diagnostic handler that needs to
be set on the Context, as well as the setDiscardValueNames() setting.

llvm-svn: 279532

llvm/lib/LTO/LTO.cpp

@@ -468,7 +468,7 @@ public:
                           const FunctionImporter::ImportMapTy &ImportList,
                           const GVSummaryMapTy &DefinedGlobals,
                           MapVector<StringRef, MemoryBufferRef> &ModuleMap) {
-    LLVMContext BackendContext;
+    LTOLLVMContext BackendContext(Conf);
 
     ErrorOr<std::unique_ptr<Module>> MOrErr =
         parseBitcodeFile(MBRef, BackendContext);

llvm/lib/LTO/LTOBackend.cpp

@@ -271,6 +271,8 @@ Error lto::thinBackend(Config &Conf, unsigned Task, AddOutputFn AddOutput,
     return Error();
 
   auto ModuleLoader = [&](StringRef Identifier) {
+    assert(Mod.getContext().isODRUniquingDebugTypes() &&
+           "ODR Type uniquing shoudl be enabled on the context");
     return std::move(getLazyBitcodeModule(MemoryBuffer::getMemBuffer(
                                               ModuleMap[Identifier], false),
                                           Mod.getContext(),