Fix -var-list-children command (MI)

This patch incldues the following:
# Add from/to arguments in -var-list-children command (MI)
## Handle from and to args
## Don't print children=[] if numchild is 0
## A bit refactoring
## Add MiVarTestCase.test_lldbmi_var_list_children test
# Refactor -var-list-children (MI)
## Remove CMICmdCmdVarListChildren::VecMIValueResult_t
## Add CMICmdCmdVarListChildren::m_miValueList
## Remove CMICmdCmdVarListChildren::m_vecMiValueResult
   (use CMICmdCmdVarListChildren::m_miValueList instead)
## Print error message if value is invalid:
   ```
     ^error,msg="variable invalid"
   ```
## Refactor CMICmdCmdVarListChildren::Acknowledge and remove duplicated code
## Don't print children=[] if numchild is 0
## Add error checking
## Use CMICmnLLDBDebugSessionInfo::VariableInfoFormat_e for print-values
## Add few more empty-range test cases in MiVarTestCase.test_lldbmi_var_list_children
#Improve MiVarTestCase.test_lldbmi_var_list_children test (MI)

llvm-svn: 236820