layers: Fix invalid push constant checks (GH953)

layers: Fix invalid push constant checks (GH953)

Rework of invalid push constant checks, motivated mostly
by some spec updates and prior bad assumptions.

- Add check/test for a shader stage flag being set more
  than once in CreatePipelineLayout.
- Remove checks and tests having to do with warning about
  overlapping ranges in CreatePipelineLayout.  Ranges can
  certainly overlap when used in different stages.  And
  the prohibition of more than one range with the same
  stage flag removes the possibility of overlap in the same
  stage.  These checks were warnings and so probably not
  noticed.

Fixes #953
Finishes VU 00871

Change-Id: Icc9d3d84fa3132f266075f2f1d45c243e9e1a65f