[clang-rename] Enforce LLVM policy about braces around single line control flow statement body.

Although it is not explicitly stated in LLVM Coding Standards, LLVM developers
prefer to omit braces around flow control statements with single line body.

For example the following piece of code

```
if (condition)
  std::cout << "Hello, world!" << std::endl;
```

is preferred to

```
if (condition) {
  std::cout << "Hello, world!" << std::endl;
}
```

So far clang-rename has ignored this. This patch makes clang-rename more
"LLVM-ish".

llvm-svn: 280640