[clang-tidy] Store checks profiling info as JSON files

[clang-tidy] Store checks profiling info as JSON files

Summary:
Continuation of D46504.

Example output:
```
  $ clang-tidy -enable-check-profile -store-check-profile=. -checks=-*,readability-function-size source.cpp
  $ # Note that there won't be timings table printed to the console.
  $ cat *.json
  {
  "file": "/path/to/source.cpp",
  "timestamp": "2018-05-16 16:13:18.717446360",
  "profile": {
    "time.clang-tidy.readability-function-size.wall": 1.0421266555786133e+00,
    "time.clang-tidy.readability-function-size.user": 9.2088400000005421e-01,
    "time.clang-tidy.readability-function-size.sys": 1.2418899999999974e-01
  }
  }
```

There are two arguments that control profile storage:

* `-store-check-profile=<prefix>`

  By default reports are printed in tabulated format to stderr. When this option
  is passed, these per-TU profiles are instead stored as JSON.
  If the prefix is not an absolute path, it is considered to be relative to the
  directory from where you have run :program:`clang-tidy`. All `.` and `..`
  patterns in the path are collapsed, and symlinks are resolved.

  Example:
  Let's suppose you have a source file named `example.cpp`, located in
  `/source` directory.

  * If you specify `-store-check-profile=/tmp`, then the profile will be saved
    to `/tmp/<timestamp>-example.cpp.json`

  * If you run :program:`clang-tidy` from within `/foo` directory, and specify
    `-store-check-profile=.`, then the profile will still be saved to
    `/foo/<timestamp>-example.cpp.json`

Reviewers: alexfh, sbenza, george.karpenkov, NoQ, aaron.ballman

Reviewed By: alexfh, george.karpenkov, aaron.ballman

Subscribers: Quuxplusone, JonasToth, aaron.ballman, llvm-commits, rja, Eugene.Zelenko, xazax.hun, mgrang, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D46602

llvm-svn: 334101