[Support] Add JSON streaming output API, faster where the heavy value types aren't needed.

Summary:
There's still a little bit of constant factor that could be trimmed (e.g.
more overloads to avoid round-tripping primitives through json::Value).
But this solves the memory scaling problem, and greatly improves the performance
constant factor, and the API should leave room for optimization if needed.

Adapt TimeProfiler to use it, eliminating almost all the performance regression
from r358476.

Performance test on my machine:
perf stat -r 5 ~/llvmbuild-opt/bin/clang++ -w -S -ftime-trace -mllvm -time-trace-granularity=0 spirit.cpp

Handcrafted JSON (HEAD=r358532 with r358476 reverted): 2480ms
json::Value (HEAD): 2757ms (+11%)
After this patch: 2520 ms (+1.6%)

Reviewers: anton-afanasyev, lebedev.ri

Subscribers: kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60804

llvm-svn: 359186