[Common] Threads: use function_ref instead of std::function

[ruiu]

What is a benefit of doing this?

[MaskRay]

In D54813#1308421, @ruiu wrote:

What is a benefit of doing this?

a) Performance (very minor): it removes a heap allocation of the function constructor (template<class F> function(F f))
b) Clarity: it suggests that the callable's lifetime should end after the callee returns. Such callback is widely used in llvm. lld also uses it a lot (at least 17 times).

[ruiu]

LGTM

include/lld/Common/Threads.h

 template <typename R, class FuncTy> void parallelForEach(R &&Range, FuncTy Fn) {
   if (ThreadsEnabled)
     for_each(llvm::parallel::par, std::begin(Range), std::end(Range), Fn);
   else
     for_each(llvm::parallel::seq, std::begin(Range), std::end(Range), Fn);
 }
 
 inline void parallelForEachN(size_t Begin, size_t End,
-                             std::function<void(size_t)> Fn) {
+                             llvm::function_ref<void(size_t)> Fn) {
   if (ThreadsEnabled)
     for_each_n(llvm::parallel::par, Begin, End, Fn);
   else
     for_each_n(llvm::parallel::seq, Begin, End, Fn);
 }
 
 } // namespace lld
 
 #endif