Fix #2005. Allow multiple compilation units to declare identical push_constant blocks (#2123)

* Fixes #2005

Allow multiple units in a stage to have push_constants as long
as the blocks match.
Requires #2006 to be fixed to be functional.

* tweaks to #2005 fix after some testing

* add unit tests for push constants across multiple compilation units

For #2005

* update reference output for tests that fail validation

* fix uninitialized result.validationResult

Test/link.vk.differentPC.1.0.frag

@@ -0,0 +1,16 @@
+#version 450
+
+layout (push_constant) uniform PushConstantBlock
+{
+	vec4 color;
+	vec4 color2;
+	float scale;
+	float scale2;
+} uPC;
+
+float
+getScale()
+{
+	return uPC.scale;
+}
+