[clang-format] Fix breaking of qualified operator

Summary:
From https://bugs.llvm.org/show_bug.cgi?id=40516
```
$ cat a.cpp
const NamespaceName::VeryLongClassName &NamespaceName::VeryLongClassName::myFunction() {
  // do stuff
}

const NamespaceName::VeryLongClassName &NamespaceName::VeryLongClassName::operator++() {
  // do stuff
}
$ ~/ll/build/opt/bin/clang-format -style=LLVM a.cpp
const NamespaceName::VeryLongClassName &
NamespaceName::VeryLongClassName::myFunction() {
  // do stuff
}

const NamespaceName::VeryLongClassName &NamespaceName::VeryLongClassName::
operator++() {
  // do stuff
}
```
What was happening is that the split penalty before `operator` was being set to
a smaller value by a prior if block. Moved checks around to fix this and added a
regression test.

Reviewers: djasper

Reviewed By: djasper

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57604

llvm-svn: 353033