[clang-format] RFC add SeparateDeclarationBlocks: (Leave|Always)

we have SeparateDefinitionBlocks which puts a newline between function definitions

```c++
void foo()
{
}
<----- newline added
void bar()
{
}
```

what do you think about having a SeparateDeclarationBlocks for putting newlines after declarations, I func headers with large amount of doxygen style comments get visually cluttered and would like to be able to auto add a newline.

```c++
/** foo func */
void foo();
/** bar func */
void bar();
```

becomes
```c++
/** foo func */
void foo();

/** bar func */
void bar();
```

@owenca , @HazardyKnusperkeks, I would be interested to hear your opinion on the value of such a capability.

Start by tracing clang-format's existing SeparateDefinitionBlocks option and its formatting behavior. Determine where declaration blocks are classified, then verify that the requested setting inserts a blank line between documented declarations without changing other formatting; the examples in the issue define the expected result.

Written by the indexing model from the issue text.