Add ability to update existing variables with values read from the command line

Add ability to update existing variables with values read from the command line
to certain classes.  This is nice because it means that in header files we can
just declare a value, and still have that value be set based on a command-line
argument.  The difference is now that the #include of CommandLine.h does not
need to go into the header file as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2708 91177308-0d34-0410-b5e6-96231b3b80d8