This allows us to keep a separate symbol type for each field in a
struct. This can allow us to assign internal names to struct types.
It could also allow us to add internal fields to user defined stucts.

Bug: angleproject:2665
Change-Id: I6a129107d9db66c54b98b07684c3ead5801712ba
Reviewed-on: https://chromium-review.googlesource.com/1101565Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>