Add "watch set" command as a more general interface in conjunction with "frame var -w".

Also add test cases for watching a variable as well as a location expressed as an expression.

o TestMyFirstWatchpoint.py:

  Modified to test "watchpoint set -w write global".

o TestWatchLocationWithWatchSet.py:

  Added to test "watchpoint set -w write -x 1 g_char_ptr + 7" where a contrived example program
  with several threads is supposed to only access the array index within the range [0..6], but
  there's some misbehaving thread writing past the range.

rdar://problem/10701761

llvm-svn: 149280