Add an overload of SetValueFromCString() to ValueObjectSynthetic that is just...

Add an overload of SetValueFromCString() to ValueObjectSynthetic that is just a passthrough to the parent object

Without this overload, attempts to edit the value of a variable with synthetic children enabled would change the value inside the synthetic ValueObject, but not propagate the changes to the underlying storage, hence resulting in no write for any meaningful purpose

Comes with a test case, and fixes rdar://19586311

llvm-svn: 227120