Don't compile usage of std::thread

As of the time of this writing it's not actually used anywhere meaningfullly
throughout the LLVM repo that we need, and it unfortunately uses `std::thread`
which isn't available in mingw-w64 toolchains with the win32 threading model
(the one that we use).

Two major changes were made to achieve this:

1. The `ThreadPool.cpp` file was just entirely commented out. This isn't used
   anywhere in the LLVM repo nor in Rust itself.
2. The `ParallelCG.cpp` file was mostly deleted. Unfortunately it's used a few
   places in LLVM and is needed to link correctly, but we in Rust don't use it
   at all. For now it's just a stub implementation that hopefully compiles
   everywhere, but perhaps we can find a less invasive (aka doesn't have rebase
   conflicts in the future) change to apply soon.

For reference, the upstream LLVM bug has been reported [1]

[1]: https://llvm.org/bugs/show_bug.cgi?id=26365

include/llvm/Support/ThreadPool.h

@@ -11,6 +11,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if 0
+
 #ifndef LLVM_SUPPORT_THREAD_POOL_H
 #define LLVM_SUPPORT_THREAD_POOL_H
 
@@ -134,3 +136,5 @@ class ThreadPool {
 }
 
 #endif // LLVM_SUPPORT_THREAD_POOL_H
+
+#endif

include/llvm/Support/thread.h

@@ -14,6 +14,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if 0
+
 #ifndef LLVM_SUPPORT_THREAD_H
 #define LLVM_SUPPORT_THREAD_H
 
@@ -64,3 +66,5 @@ struct thread {
 #endif // LLVM_ENABLE_THREADS
 
 #endif
+
+#endif

lib/CodeGen/ParallelCG.cpp

@@ -25,72 +25,11 @@
 
 using namespace llvm;
 
-static void codegen(Module *M, llvm::raw_pwrite_stream &OS,
-                    const Target *TheTarget, StringRef CPU, StringRef Features,
-                    const TargetOptions &Options, Reloc::Model RM,
-                    CodeModel::Model CM, CodeGenOpt::Level OL,
-                    TargetMachine::CodeGenFileType FileType) {
-  std::unique_ptr<TargetMachine> TM(TheTarget->createTargetMachine(
-      M->getTargetTriple(), CPU, Features, Options, RM, CM, OL));
-
-  legacy::PassManager CodeGenPasses;
-  if (TM->addPassesToEmitFile(CodeGenPasses, OS, FileType))
-    report_fatal_error("Failed to setup codegen");
-  CodeGenPasses.run(*M);
-}
-
 std::unique_ptr<Module>
 llvm::splitCodeGen(std::unique_ptr<Module> M,
                    ArrayRef<llvm::raw_pwrite_stream *> OSs, StringRef CPU,
                    StringRef Features, const TargetOptions &Options,
                    Reloc::Model RM, CodeModel::Model CM, CodeGenOpt::Level OL,
                    TargetMachine::CodeGenFileType FileType) {
-  StringRef TripleStr = M->getTargetTriple();
-  std::string ErrMsg;
-  const Target *TheTarget = TargetRegistry::lookupTarget(TripleStr, ErrMsg);
-  if (!TheTarget)
-    report_fatal_error(Twine("Target not found: ") + ErrMsg);
-
-  if (OSs.size() == 1) {
-    codegen(M.get(), *OSs[0], TheTarget, CPU, Features, Options, RM, CM,
-            OL, FileType);
-    return M;
-  }
-
-  std::vector<thread> Threads;
-  SplitModule(std::move(M), OSs.size(), [&](std::unique_ptr<Module> MPart) {
-    // We want to clone the module in a new context to multi-thread the codegen.
-    // We do it by serializing partition modules to bitcode (while still on the
-    // main thread, in order to avoid data races) and spinning up new threads
-    // which deserialize the partitions into separate contexts.
-    // FIXME: Provide a more direct way to do this in LLVM.
-    SmallVector<char, 0> BC;
-    raw_svector_ostream BCOS(BC);
-    WriteBitcodeToFile(MPart.get(), BCOS);
-
-    llvm::raw_pwrite_stream *ThreadOS = OSs[Threads.size()];
-    Threads.emplace_back(
-        [TheTarget, CPU, Features, Options, RM, CM, OL, FileType,
-         ThreadOS](const SmallVector<char, 0> &BC) {
-          LLVMContext Ctx;
-          ErrorOr<std::unique_ptr<Module>> MOrErr =
-              parseBitcodeFile(MemoryBufferRef(StringRef(BC.data(), BC.size()),
-                                               "<split-module>"),
-                               Ctx);
-          if (!MOrErr)
-            report_fatal_error("Failed to read bitcode");
-          std::unique_ptr<Module> MPartInCtx = std::move(MOrErr.get());
-
-          codegen(MPartInCtx.get(), *ThreadOS, TheTarget, CPU, Features,
-                  Options, RM, CM, OL, FileType);
-        },
-        // Pass BC using std::move to ensure that it get moved rather than
-        // copied into the thread's context.
-        std::move(BC));
-  });
-
-  for (thread &T : Threads)
-    T.join();
-
-  return {};
+  return M;
 }

lib/Support/ThreadPool.cpp

@@ -11,6 +11,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if 0
+