Fix a crasher in StackFrame::GetValueForVariableExpressionPath()

There was a crash that would happen if an IDE would ask for a child of a shared pointer via any SB API call that ends up calling StackFrame::GetValueForVariableExpressionPath(). The previous code expects an error to be set describing why the synthetic child of a type was not able to be found, but we have some synthetic child providers that weren't setting the error and returning an empty value object shared pointer. This fixes that to ensure we don't lose our debug session by crashing, fully tests GetValueForVariableExpressionPath functionality, and ensures we don't crash on GetValueForVariableExpressionPath() in the future.

Differential Revision: https://reviews.llvm.org/D59200

llvm-svn: 355850