Add a -V <bool> flag to frame variable/expression that enables execution of type validators. The jury is still out on what the user experience of type validators should be, so for now gate it on a specific flag. The mode I am using is prefix variables that fail to validate with a bang, and then emitting the actual validation error on a separate line. Of course, given the total absence of validators, this should never actually happen to you

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217303 91177308-0d34-0410-b5e6-96231b3b80d8