Implement SPV_KHR_post_depth_coverage

Added support for both extension GL_ARB_post_depth_coverage and GL_EXT_post_depth_coverage.

Test/spv.extPostDepthCoverage_Error.frag

@@ -0,0 +1,9 @@
+#version 450
+
+#extension GL_EXT_post_depth_coverage : enable
+
+layout(post_depth_coverage) in; // should fail since for GL_EXT_post_depth_coverage
+                                // explicit declaration of early_fragment_tests is required
+
+void main () {
+}