[DTLTO][LLVM] Make the ThinLTO backend wait() method virtual

The DTLTO ThinLTO backend will override this function to perform
code generation.

The DTLTO ThinLTO backend will not do any codegen when invoked for each
task. Instead, it will generate the required information (e.g., the
summary index shard, import list, etc.) to allow for the codegen to be
performed externally. The backend's `wait` function will then invoke an
external distributor process to do backend compilations.

Author: bd1976bris

llvm/include/llvm/LTO/LTO.h

@@ -230,7 +230,7 @@ class ThinBackendProc {
       const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR,
       MapVector<StringRef, BitcodeModule> &ModuleMap,
       DenseMap<StringRef, std::string> &ModuleTriples) = 0;
-  Error wait() {
+  virtual Error wait() {
     BackendThreadPool.wait();
     if (Err)
       return std::move(*Err);