Moving style option formatting to libFormat

The help text for clang-format's -style option and the function that processes
its value is moved to libFormat in this patch. The goal is to enable other
tools that use libFormat and also have a -style option to behave consistently
with clang-format.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191666 91177308-0d34-0410-b5e6-96231b3b80d8