clang-format: add option to merge empty function body

Summary:
This option supplements the AllowShortFunctionsOnASingleLine flag, to
merge empty function body at the beginning of the line: e.g. when the
function is not short-enough and breaking braces after function.

  int f()
  {}

Reviewers: krasimir, djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D33447

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305272 91177308-0d34-0410-b5e6-96231b3b80d8