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).

llvm-svn: 194090