SPV: Fix #904: Correctly check for built-in block redeclations for location check.

Test/spv.noLocation.vert

@@ -1,11 +1,11 @@
 #version 450
 
 layout(location = 1) in vec4 in1;
-in vec4 in2;
+in vec4 in2;                        // ERROR
 layout(location = 3) in vec4 in3;
 
 layout(location = 1) out vec4 out1;
-out vec4 out2;
+out vec4 out2;                      // ERROR
 layout(location = 3) out vec4 out3;
 
 layout(location = 10) out inb1 { 
@@ -16,18 +16,24 @@ out inb2 {
     layout(location = 12) vec4 a;
     layout(location = 13) vec4 b;
 } inbi2;
-out inb3 {
+out inb3 {                          // ERROR
     vec4 a;
     vec4 b;
 } inbi3;
 
 layout(location = 14) out struct S1 { vec4 a; } s1;
-out struct S2 { vec4 a; } s2;
+out struct S2 { vec4 a; } s2;       // ERROR
 
 struct SS { int a; };
 out layout(location = 15) SS ss1;
-out SS ss2;
+out SS ss2;                         // ERROR
+
+out gl_PerVertex {
+    vec4 gl_Position;
+    float gl_ClipDistance[2];
+};
 
 void main()
 {
+    gl_ClipDistance[0] = 1.0;
 }