clang-format: Allow line merging and partial formatting of nested blocks

Before, clang-format would always format entire nested blocks, which
can be unwanted e.g. for long DEBUG({...}) statements. Also
clang-format would not allow to merge lines in nested blocks (e.g. to
put "if (a) return;" on one line in Google style).

This is the first step of several refactorings mostly focussing on the
additional functionality (by reusing the "format many lines" code to
format the children of a nested block). The next steps are:
* Pull out the line merging into its own class.
* Seperate the formatting of many lines from the formatting of a single
  line (and the analysis of the solution space).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194090 91177308-0d34-0410-b5e6-96231b3b80d8
Documentation tweaks: Fix a typo in a reference to the C++ standard,
another typo in a comment, and update formatting of some comments to
use Doxygen \brief annotations instead of repeating the name of the
documented entity.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194092 91177308-0d34-0410-b5e6-96231b3b80d8