Make formatting of empty blocks more consistent.

With this patch, the simplified rule is:
If the block is part of a declaration (class, namespace, function,
enum, ..), merge an empty block onto a single line. Otherwise
(specifically for the compound statements of if, for, while, ...),
keep the braces on two separate lines.

The reasons are:
- Mostly the formatting of empty blocks does not matter much.
- Empty compound statements are really rare and are usually just
  inserted while still working on the code. If they are on two lines,
  inserting code is easier. Also, overlooking the "{}" of an
  "if (...) {}" can be really bad.
- Empty declarations are not uncommon, e.g. empty constructors. Putting
  them on one line saves vertical space at no loss of readability.

llvm-svn: 183008