clang-format: better handle statement macros

Summary:
Some macros are used in the body of function, and actually contain the trailing semicolon: they should thus be automatically followed by a new line, and not get merged with the next line. This is for example the case with Qt's Q_UNUSED macro:

  void foo(int a, int b) {
    Q_UNUSED(a)
    return b;
  }

This patch deals with these cases by introducing a new option to specify list of statement macros. This re-uses the system already in place for foreach macros, to ensure there is no impact on performance.

Reviewers: krasimir, djasper, klimek

Reviewed By: krasimir

Subscribers: acoomans, mgrang, alexfh, klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D33440

llvm-svn: 343602