[Support][JSON] Add reserve() to json Array

[anton-afanasyev]

Summary

Space reservation increases json lib performance for the arrays with large number of entries.
Here is an example and discussion: https://reviews.llvm.org/D60609#1468941.
In short: there is json array with ~32K entries, one can build it using llvm::json::Array by pushing back each entry.
This pushing takes ~4% of whole time compared to the method of preliminary memory reservation: (3995-3845)/3995 = 3.75%.

[lebedev.ri]

As i have previously discussed in IRC with @sammccall this change needs motivational numbers.
Just copying them (+methodology) from https://reviews.llvm.org/D60609#inline-538565 sounds good.
-5.5% sounds rather great to me.

[anton-afanasyev]

In D60788#1469005, @lebedev.ri wrote:

As i have previously discussed in IRC with @sammccall this change needs motivational numbers.
Just copying them (+methodology) from https://reviews.llvm.org/D60609#inline-538565 sounds good.
-5.5% sounds rather great to me.

Ok, updated summary.

[sammccall]

Thanks for gathering the numbers here, it is really useful to see how people are using this in performance-sensitive contexts. Definitely makes sense to fix this.

[lebedev.ri]

@sammccall @anton-afanasyev thank you.

llvm/include/llvm/Support/JSON.h

public:
 
   iterator begin() { return V.begin(); }
   const_iterator begin() const { return V.begin(); }
   iterator end() { return V.end(); }
   const_iterator end() const { return V.end(); }
 
   bool empty() const { return V.empty(); }
   size_t size() const { return V.size(); }
+  void reserve(size_t S) { V.reserve(S); }
 
   void clear() { V.clear(); }
   void push_back(const Value &E) { V.push_back(E); }
   void push_back(Value &&E) { V.push_back(std::move(E)); }
   template <typename... Args> void emplace_back(Args &&... A) {
     V.emplace_back(std::forward<Args>(A)...);
   }
   void pop_back() { V.pop_back(); }