layers: Make DescriptorSetLayout robust for CTS

CTS sends an invalid descriptor set that the validation tracking was not
robust to -- two binding with identical binding ID values.  After
reporting the invalid input data, the test application could crash.
Validation internal data is now created in a way that invalid data
should less easily cause program termination.

Change-Id: Ib84a5234267f7bece9be377badd22696e3633b1b