GLSL: Implement GL_EXT_control_flow_attributes.

See https://github.com/KhronosGroup/GLSL/pull/11.

Test/spv.controlFlowAttributes.frag

@@ -0,0 +1,39 @@
+#version 450
+
+#extension GL_EXT_control_flow_attributes : enable
+
+bool cond;
+
+void main()
+{
+        [[unroll]]                 for (int i = 0; i < 8; ++i) { }
+        [[loop]]                   for (;;) { }
+        [[dont_unroll]]            while(true) {  }
+        [[dependency_infinite]]    do {  } while(true);
+        [[dependency_length(1+3)]] for (int i = 0; i < 8; ++i) { }
+        [[flatten]]                if (cond) { } else { }
+        [[branch]]                 if (cond) cond = false;
+        [[dont_flatten]]           switch(3) {  }                      // dropped
+        [[dont_flatten]]           switch(3) { case 3: break; }
+
+        // warnings on all these
+        [[unroll(2)]]              for (int i = 0; i < 8; ++i) { }
+        [[dont_unroll(-2)]]        while(true) {  }
+        [[dependency_infinite(3)]] do {  } while(true);
+        [[dependency_length]]      for (int i = 0; i < 8; ++i) { }
+        [[flatten(3)]]             if (cond) { } else { }
+        [[branch(5.2)]]            if (cond) cond = false;
+        [[dont_flatten(3 + 7)]]    switch(3) { case 3: break; }
+
+        // other valid uses
+        [[ unroll, dont_unroll, dependency_length(2) ]]  while(cond) {  }
+        [ [ dont_flatten , branch ] ]                    switch(3) { case 3: break; }
+        [
+            // attribute
+            [
+                // here
+                flatten
+            ]
+        ]                       if (cond) { } else { }
+        [[ dependency_length(2), dependency_infinite ]]  while(cond) {  }
+}