HLSL: Flatten a return struct from an entry point and assign locations after flattening.

Locations now get assigned in order, but skipping built-ins, which can be
done post flattening.

Test/baseResults/hlsl.intrinsics.barriers.comp.out

@@ -18,7 +18,7 @@ local_size = (1, 1, 1)
 0:11            0.000000
 0:11        Branch: Return
 0:?   Linker Objects
-0:?     '@entryPointOutput' (out float)
+0:?     '@entryPointOutput' (layout(location=0 ) out float)
 
 
 Linked compute stage:
@@ -43,7 +43,7 @@ local_size = (1, 1, 1)
 0:11            0.000000
 0:11        Branch: Return
 0:?   Linker Objects
-0:?     '@entryPointOutput' (out float)
+0:?     '@entryPointOutput' (layout(location=0 ) out float)
 
 // Module Version 10000
 // Generated by (magic number): 80001