Pull in r270240 from upstream clang trunk (by me):

Pull in r270240 from upstream clang trunk (by me):

  Make __FreeBSD_cc_version predefined macro configurable at build time

  The `FreeBSDTargetInfo` class has always set the `__FreeBSD_cc_version`
  predefined macro to a rather static value, calculated from the major OS
  version.

  In the FreeBSD base system, we will start incrementing the value of this
  macro whenever we make any signifant change to clang, so we need a way
  to configure the macro's value at build time.

  Use `FREEBSD_CC_VERSION` for this, which we can define in the FreeBSD
  build system using either the `-D` command line option, or an include
  file.  Stock builds will keep the earlier value.

  Differential Revision: http://reviews.llvm.org/D20037

Follow-up commits will start using the __FreeBSD_cc_version to determine
whether a bootstrap compiler has to be built during buildworld.