clang-format: Add special case to reduce indentaiton in streams.

This is similar to how we treat assignments and seems to be generally
desirable.

Before:
  llvm::errs() << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
                      aaaaaaaaaaaaaaaaaaaaaaaaaaaa,
                      aaaaaaaaaaaaaaaaaaaaaaaaaaaa);

After:
  llvm::errs() << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
      aaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaa);

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@206384 91177308-0d34-0410-b5e6-96231b3b80d8