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.16BitAccess.comp

@@ -0,0 +1,18 @@
+#version 430 core
+
+#extension GL_EXT_shared_memory_block : enable
+#extension GL_EXT_shader_explicit_arithmetic_types: enable
+
+layout(local_size_x = 2) in;
+
+shared first
+{
+    int16_t a;
+    float16_t f;
+};
+
+void main()
+{
+    a = int16_t(3);
+    f = float16_t(12.3);
+}