[DTLTO][LLVM] Add a setup() member to the ThinLTO backends

The new setup() member is now called to allow the ThinLTO backends to
prepare for code generation.

For the DTLTO backend, this will be used to preallocate storage for the
information required to perform the backend compilation jobs.

Author: bd1976bris

llvm/include/llvm/LTO/LTO.h

@@ -225,6 +225,7 @@ class ThinBackendProc {
         BackendThreadPool(ThinLTOParallelism) {}
 
   virtual ~ThinBackendProc() = default;
+  virtual void setup(unsigned MaxTasks) {}
   virtual Error start(
       unsigned Task, BitcodeModule BM,
       const FunctionImporter::ImportMapTy &ImportList,

llvm/lib/LTO/LTO.cpp

@@ -2036,6 +2036,8 @@ Error LTO::runThinLTO(AddStreamFn AddStream, FileCache Cache,
           ResolvedODR[Mod.first], ThinLTO.ModuleMap, ThinLTO.ModuleTriples);
     };
 
+    BackendProcess->setup(ModuleMap.size());
+
     if (BackendProcess->getThreadCount() == 1 ||
         BackendProcess->isSensitiveToInputOrder()) {
       // Process the modules in the order they were provided on the