Add experimental flag for adaptive parameter bin-packing.

This is not activated for any style, might change or go away
completely.

For those that want to play around with it, set
ExperimentalAutoDetectBinPacking to true.

clang-format will then:
Look at whether function calls/declarations/definitions are currently
formatted with one parameter per line (on a case-by-case basis). If so,
clang-format will avoid bin-packing the parameters. If all parameters
are on one line (thus that line is "inconclusive"), clang-format will
make the choice dependent on whether there are other bin-packed
calls/declarations in the same file.

The reason for this change is that bin-packing in some situations can be
really bad and an author might opt to put one parameter on each line. If
the author does that, he might want clang-format not to mess with that.
If the author is unhappy with the one-per-line formatting, clang-format
can easily be convinced to bin-pack by putting any two parameters on the
same line.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186003 91177308-0d34-0410-b5e6-96231b3b80d8