Use braces in multi-statement DEBUG() code [NFC]

By adding braces into the DEBUG statement we can make clang-format format code
such as:

  DEBUG(stmt1(); stmt2())

as multi-line code:

  DEBUG({
    stmt1();
    stmt2();
  });

This makes control-flow in debug statements easier to read.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@220441 91177308-0d34-0410-b5e6-96231b3b80d8