Fix non statically used fragment input structs on HLSL Add static use information to struct fields that mirrors the static use information on the struct itself. This way dynamically generated HLSL doesn't need special handling for initializing fragment inputs if they are structs. This fixes a problem with the previous code where dynamically generated HLSL ended up trying to initialize structs that are not declared in the HLSL output because they were not being referenced. BUG=angleproject:2104 TEST=angle_end2end_tests Change-Id: I21283ce4fe26515d62d95e61f8155dc9a9b44cf1