[clang-tidy] Reduce false-positive ratio in misc-suspicious-missing-comma check.

Summary:
This patch is adding detection of common string literal patterns
that should not trigger warnings.

  [*] Add a limit on the number of concatenated token,
  [*] Add support for parenthese sequence of tokens,
  [*] Add detection of valid indentation.

As an example, this code will no longer trigger a warning:
```
const char* Array[] = {
  "first literal"
    "indented literal"
    "indented literal",
  "second literal",
  [...]
```

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18695

llvm-svn: 265303