After discussions with Jim and Greg, modify the 'watchpoint set' command to...

After discussions with Jim and Greg, modify the 'watchpoint set' command to become a mutiword command
with subcommand 'expression' and 'variable'.  The first subcommand is for supplying an expression to
be evaluated into an address to watch for, while the second is for watching a variable.

'watchpoint set expression' is a raw command, which means that you need to use the "--" option terminator
to end the '-w' or '-x' option processing and to start typing your expression.

Also update several test cases to comply and add a couple of test cases into TestCompletion.py,
in particular, test that 'watchpoint set ex' completes to 'watchpoint set expression ' and that
'watchpoint set var' completes to 'watchpoint set variable '.

llvm-svn: 150109