In many places in ANGLE we need to traverse a ShaderVariable tree. We
do this to store uniform offset and other information, to flatten the
tree of uniforms for the front-end, or to produce active variable lists
for uniform and shader storage blocks. In each case, we would write
separate tree traversal code.

This patch introduces a shared visitor pattern for all of the shader
variable tree traversal instances. With that get more common code. Also
it is easier to write new variable traversals. ProgramD3D and
ProgramLinkedResources in particular get nice simplificiations.

The visitor object recieves callbacks from the traversal when entering
structs, array elements, and new variables. The visitor can treat these
differently depending on the use case. A common visitor that constructs
full variable names is used as a base class in several places.

Also moves the 'isRowMajorLayout' from sh::InterfaceBlockField to
sh::ShaderVariable. This allows us to forgo using templates in several
call sites.

Bug: angleproject:3024
Change-Id: I472d81ec775e2eee92fb3d2eb0ca83860221ba2e
Reviewed-on: https://chromium-review.googlesource.com/c/1358722
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>