GLSL Linker: Track entry point across compilation units.

This wasn't needed until the recent generalization of "main" to "entry point",
so makes some HLSL-specific code be generic now, for GLSL functional correctness.

Test/baseResults/mains1.frag.out

@@ -48,6 +48,7 @@ ERROR: Linking geometry stage: At least one shader must specify a layout(max_ver
 
 Linked fragment stage:
 
+ERROR: Linking fragment stage: can't handle multiple entry points per stage
 ERROR: Linking fragment stage: Multiple function bodies in multiple compilation units for the same signature in the same stage:
     main(