clang-format: Improve selective formatting of nested statements.

Previously, clang-format could create quite corrupt formattings if
individual lines of nested blocks (e.g. in "DEBUG({})" or lambdas) were
used. With this patch, it tries to extend the formatted regions to leave
around some reasonable format without always formatting the entire
surrounding statement.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195925 91177308-0d34-0410-b5e6-96231b3b80d8