[clamg-format] Add `BreakIfInitializer` option

[SunBlack]

Since C++17 it is allowed to add a init-statement within an if condition.

It would therefore be nice if you could control whether this statement should be in an extra line or not.

So there should be 3 options:

• Keep => Don't adjust the code
• Always:

  if (const auto it = map.find(key);
      it != map.end())
  {
    ...
  }

• Never:

  if (const auto it = map.find(key); it != map.end())
  {
    ...
  }

[llvmbot]

@llvm/issue-subscribers-clang-format

Author: SunBlack (SunBlack)

Since C++17 it is allowed to add a init-statement within an [if condition](https://en.cppreference.com/w/cpp/language/if.html).

It would therefore be nice if you could control whether this statement should be in an extra line or not.

So there should be 3 options:

• Keep => Don't adjust the code
• Always:

  if (const auto it = map.find(key);
      it != map.end())
  {
    ...
  }

• Never:

  if (const auto it = map.find(key); it != map.end())
  {
    ...
  }

[SunBlack]

Since switch statements have the same init-statement variant, the question would be whether to offer 2 separate options or one common one.