Fix bugs around handling C++11 attributes.

Previously, we would format:
  int a() { ... }
  [[unused]] int b() { ... }
as...
  int a() {} [[unused] int b() {}
Now we correctly format each on its own line.

Similarly, we would detect:
  [[unused]] int b() { return 42; }
As a lambda and leave it on a single line, even if that was disallowed
by the format style.

llvm-svn: 329816