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.templatetypes.frag.out

@@ -246,7 +246,7 @@ gl_FragCoord origin is upper left
 0:45            0.000000
 0:45        Branch: Return
 0:?   Linker Objects
-0:?     '@entryPointOutput' (out float)
+0:?     '@entryPointOutput' (layout(location=0 ) out float)
 
 
 Linked fragment stage:
@@ -499,7 +499,7 @@ gl_FragCoord origin is upper left
 0:45            0.000000
 0:45        Branch: Return
 0:?   Linker Objects
-0:?     '@entryPointOutput' (out float)
+0:?     '@entryPointOutput' (layout(location=0 ) out float)
 
 // Module Version 10000
 // Generated by (magic number): 80001