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

Summary:
Space reservation increases json lib performance for the arrays with large number of entries.
Here is the example and discussion: https://reviews.llvm.org/D60609#1468941

Reviewers: lebedev.ri, sammccall

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60788

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358520 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed