HLSL: Register all entry-point in/out as part of the interface.

This makes the interface be invariant, whether or not individual
variables are used.

Test/baseResults/hlsl.semicolons.frag.out

@@ -25,6 +25,7 @@ gl_FragCoord origin is upper left
 0:17          'ps_output' (temp structure{temp 4-component vector of float color})
 0:17        Branch: Return
 0:?   Linker Objects
+0:?     '@entryPointOutput' (out structure{temp 4-component vector of float color})
 
 
 Linked fragment stage:
@@ -56,6 +57,7 @@ gl_FragCoord origin is upper left
 0:17          'ps_output' (temp structure{temp 4-component vector of float color})
 0:17        Branch: Return
 0:?   Linker Objects
+0:?     '@entryPointOutput' (out structure{temp 4-component vector of float color})
 
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