Add a .clang-format file to enhance formatting experience with clang-format

clang-format is a handy tool that formats code very intelligently.  I'd
like to use it with LLDB but it requires a .clang-format file to inform
it about LLDB-specific formatting rules.

More information on these rules are here:
http://clang.llvm.org/docs/ClangFormatStyleOptions.html

Differential Revision: http://reviews.llvm.org/D4630