This helps to detect and report parsing errors better.
The patch follows the ideas of LLDB's patches D59370 and D59381.
It adds tests for valid and some invalid cases. More checks and
tests to come. Note that the patch fixes validation of the Length
field because the value does not include the field itself.
The existing users are updated to show the error messages.
Differential Revision: https://reviews.llvm.org/D71875
