*Some more optimizations in usage of ConstString

*New setting target.max-children-count gives an upper-bound to the number of child objects that will be displayed at each depth-level
  This might be a breaking change in some scenarios. To override the new limit you can use the --show-all-children (-A) option
  to frame variable or increase the limit in your lldbinit file
*Command "type synthetic" has been split in two:
  - "type synthetic" now only handles Python synthetic children providers
  - the new command "type filter" handles filters
  Because filters and synthetic providers are both ways to replace the children of a ValueObject, only one can be effective at any given time.

llvm-svn: 137416