[Settings] Add -force flag to "settings set"

The -force option allows you to pass an empty value to settings set to
reset the value to its default. This means that the following operations
are equivalent:

  settings set -f <setting>
  settings clear <setting>

The motivation for this change is the ability to export and import
settings from LLDB. Because of the way the dumpers work, we don't know
whether a value is going to be the default or not. Hence we cannot use
settings clear and use settings set -f, potentially providing an empty
value.

Differential revision: https://reviews.llvm.org/D52772

llvm-svn: 345207