Implement SPV_KHR_post_depth_coverage

Added support for both extension GL_ARB_post_depth_coverage and GL_EXT_post_depth_coverage.

Test/spv.arbPostDepthCoverage_Error.frag

@@ -0,0 +1,12 @@
+#version 310 es
+
+#extension GL_ARB_post_depth_coverage : enable
+
+precision highp float;
+
+layout(post_depth_coverage, location = 0) in float a;  // should fail since post_depth_coverage may only
+                                                       // be declared on in only (not with variable declarations)
+
+void main () {
+
+}