APIs to GetValueAsSigned/Unsigned() in SBValue now also accept an SBError...

APIs to GetValueAsSigned/Unsigned() in SBValue now also accept an SBError parameter to give more info about any problem
The synthetic children providers now use the new (safer) APIs to get the values of objects
As a side effect, fixed an issue in ValueObject where ResolveValue() was not always updating the value before reading it

llvm-svn: 136861