Fields of nameless interface blocks are stored as TVariables and
referenced through a TIntermSymbols (unlike named blocks which use
EOpIndexDirectInterfaceBlock with a constant field indices).  With this
change, the field index is stored in the variable's type which let's the
TField of the TInterfaceBlock be directly accessible (and not need a
search by name).

This will be helpful in translation to SPIR-V as interface block members
are accessed by field index.

Bug: angleproject:4889
Change-Id: If3ab45b1e5f5f9576721dc52e2bdf1161882514f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2818242
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>