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

llvm-svn: 270240