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