[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

llvm-svn: 358520