clang-format: Fix bug in understanding string-label&value analysis.

While for <<-operators often used in log statments, a single key value
pair is always on the second operator, e.g.

  llvm::errs() << "aaaaa=" << aaaaa;

It is on the first operator for plus- or comma-concatenated strings:

  string s = "aaaaaaaaaa: " + aaaaaaaa;

(the "=" not counting because that's a different operator precedence)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290177 91177308-0d34-0410-b5e6-96231b3b80d8