[clang] [ssaf][UnsafeBufferUsage] Add JSON serialization for UnsafeBufferUsage (PR #187156)

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

> LLVM Buildbot has detected a new failure on builder `bolt-aarch64-ubuntu-clang` running on `bolt-worker-aarch64` while building `clang` at step 5 "build-clang-bolt".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/128/builds/12186
> 
> Here is the relevant piece of the build log for the reference
```
Failed Tests (1):
  Clang Perf Training :: llvm-support/build.test
```
Irrelevant 

https://github.com/llvm/llvm-project/pull/187156