The following structs are being refactored and moved into the parent
struct ShaderVariable:

VariableWithLocation
Uniform
Attribute
OutputVariable
InterfaceBlockField
Varying

Bug: angleproject:3899
Test: CQ
Change-Id: I389eb3ab4ed44a360e09fca75ecc78d64a277f83
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1785877Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>