clang-format: Improve function declaration detection.

Before:
  ReturnType MACRO
      FunctionName() {}

After:
  ReturnType MACRO
  FunctionName() {}

This fixes llvm.org/PR21404.

I wonder what the motivation for that if-condition was. But as no test
breaks, ...

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220801 91177308-0d34-0410-b5e6-96231b3b80d8