Add support for GL_EXT_shared_memory_block

Uses SPV_KHR_workgroup_memory_explicit_layout.  Note that if
GL_EXT_scalar_block_layout is enabled, Workgroup blocks can also use
scalar layout.

Test/spv.WorkgroupMemoryExplicitLayout.NonBlock.comp

@@ -0,0 +1,14 @@
+#version 430 core
+
+#extension GL_EXT_shared_memory_block : enable
+
+layout(local_size_x = 8) in;
+
+shared int a;
+shared int b;
+
+void main()
+{
+    a = 2;
+    b = 3;
+}