[clang-tidy] Add a new checker to detect missing comma in initializer list.

Summary:
This checker is able to detect missing comma in 
an array of string literals.

```
  const char* A[] = {
    "abc",
    "def"   // missing comma (no compiler warnings)
    "ghi",
  };
```

The ratio of false-positive is reduced by restricting the
size of the array considered and the ratio of missing
comma.

To validate the quantity of false positive, the checker
was tried over LLVM and chromium code and detected these
cases:

[[ http://reviews.llvm.org/D18454 | http://reviews.llvm.org/D18454 ]]
[[https://codereview.chromium.org/1807753002/ | https://codereview.chromium.org/1807753002/]]
[[https://codereview.chromium.org/1826193002/ | https://codereview.chromium.org/1826193002/]]
[[https://codereview.chromium.org/1805713002/ | https://codereview.chromium.org/1805713002/]]

Reviewers: alexfh

Subscribers: LegalizeAdulthood, szdominik, xazax.hun, cfe-commits

Differential Revision: http://reviews.llvm.org/D18457

llvm-svn: 265033