[clang-format] Add ability to wrap braces after multi-line control statements

Summary:
Change the BraceWrappingFlags' AfterControlStatement from a bool to an enum with three values:

* "Never": This is the default, and does not do any brace wrapping after control statements.
* "MultiLine": This only wraps braces after multi-line control statements (this really only happens when a ColumnLimit is specified).
* "Always": This always wraps braces after control statements.

The first and last options are backwards-compatible with "false" and "true", respectively.

The new "MultiLine" option is useful for when a wrapped control statement's indentation matches the subsequent block's indentation. It makes it easier to see at a glance where the control statement ends and where the block's code begins. For example:

```
if (
  foo
  && bar )
{
  baz();
}
```

vs.

```
if (
  foo
  && bar ) {
  baz();
}
```

Short control statements (1 line) do not wrap the brace to the next line, e.g.

```
if (foo) {
  bar();
} else {
  baz();
}
```

Reviewers: sammccall, owenpan, reuk, MyDeveloperDay, klimek

Reviewed By: MyDeveloperDay

Subscribers: MyDeveloperDay, cfe-commits

Patch By: mitchell-stellar

Tags: #clang-format, #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D68296

llvm-svn: 373647