rearchitect the registration mechanism used by the command line option stuff.

This dramatically reduce the amount of memory allocated by the commandline stuff
at static init time, changing it to build local data structures when ParseCommandLineOptions
is called.  In a dummy empty program that links some llvm libraries, this reduces
the number of malloc'd bytes from 4864 to 3360 on entry to main.  Most of that
memory is now allocated by non-commandline related stuff.

llvm-svn: 35701