[lld][COFF][ELF][WebAssembly] Replace --[no-]threads /threads[:no] with --threads={1,2,...} /threads:{1,2,...}

--no-threads is a name copied from gold.
gold has --no-thread, --thread-count and several other --thread-count-*.

There are needs to customize the number of threads (running several lld
processes concurrently or customizing the number of LTO threads).
Having a single --threads=N is a straightforward replacement of gold's
--no-threads + --thread-count.

--no-threads is used rarely. So just delete --no-threads instead of
keeping it for compatibility for a while.

If --threads= is specified (ELF,wasm; COFF /threads: is similar),
--thinlto-jobs= defaults to --threads=,
otherwise all available hardware threads are used.

There is currently no way to override a --threads={1,2,...}. It is still
a debate whether we should use --threads=all.

Reviewed By: rnk, aganea

Differential Revision: https://reviews.llvm.org/D76885

include/llvm/Support/Parallel.h

@@ -12,6 +12,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/MathExtras.h"
+#include "llvm/Support/Threading.h"
 
 #include <algorithm>
 #include <condition_variable>
@@ -33,6 +34,11 @@ struct is_execution_policy
 constexpr sequential_execution_policy seq{};
 constexpr parallel_execution_policy par{};
 
+// Strategy for the default executor used by the parallel routines provided by
+// this file. It defaults to using all hardware threads and should be
+// initialized before the first use of parallel routines.
+extern ThreadPoolStrategy strategy;
+
 namespace detail {
 
 #if LLVM_ENABLE_THREADS

lib/Support/Parallel.cpp

@@ -20,6 +20,8 @@
 #include <thread>
 #include <vector>
 
+llvm::ThreadPoolStrategy llvm::parallel::strategy;
+
 namespace llvm {
 namespace parallel {
 namespace detail {
@@ -78,6 +80,9 @@ public:
         T.join();
   }
 
+  struct Creator {
+    static void *call() { return new ThreadPoolExecutor(strategy); }
+  };
   struct Deleter {
     static void call(void *Ptr) { ((ThreadPoolExecutor *)Ptr)->stop(); }
   };
@@ -131,7 +136,8 @@ Executor *Executor::getDefaultExecutor() {
   // are more frequent with the debug static runtime.
   //
   // This also prevents intermittent deadlocks on exit with the MinGW runtime.
-  static ManagedStatic<ThreadPoolExecutor, object_creator<ThreadPoolExecutor>,
+
+  static ManagedStatic<ThreadPoolExecutor, ThreadPoolExecutor::Creator,
                        ThreadPoolExecutor::Deleter>
       ManagedExec;
   static std::unique_ptr<ThreadPoolExecutor> Exec(&(*ManagedExec));

utils/gn/secondary/lld/Common/BUILD.gn

@@ -39,7 +39,6 @@ static_library("Common") {
     "Reproduce.cpp",
     "Strings.cpp",
     "TargetOptionsCommandFlags.cpp",
-    "Threads.cpp",
     "Timer.cpp",
     "Version.cpp",
   ]