This change implements a key instruction of SPIR-V, OpAccessChain.
Inspired by glslang's implementation, a class is added (AccessChain)
that tracks "indices" into a base value.  These indices could select a
field of a block, an element of an array, a column of a matrix, or a
component of a vector.  Nuances (such as multi-component swizzle as an
lvalue not representable in SPIR-V) and optimizations (such as
all-literal indices to an rvalue) that are implemented in glslang have
also been implemented in this change.  As a result, this change
implements all manners of loads and stores (with the exception of this
gotcha: https://github.com/KhronosGroup/glslang/issues/2637).

The change uses this feature to translate the basic shader which does
`gl_Position = positionAttr;` (by implementing EOpAssign), and enables a
test that uses this shader.

Bug: angleproject:4889
Change-Id: I22dbe5b169ce499eaac657902164aca3b0ebc193
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2911880
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>