Vulkan: Move mWriteDescriptors out of DescriptorSetDescBuilder

DescriptorSetDescBuilder has two data structures: mWriteDescriptors,
which keeps track of "layout" of descriptorSet, and mDescriptorInfos,
which is the actual cache key for descriptorSet. mDescriptorInfos will
be updated every time buffer or image changes. But mWriteDescriptors is
immutable with buffer/image, it is static per program executable (with
exception of InputAttachment). Right now whenever there is a buffer or
image change, we call into DescriptorSetDescBuilder and update both
mWriteDescriptors and mDescriptorInfos. This CL moves mWriteDescriptors
out of DescriptorSetDescBuilder, and stores it in ProgramExecutableVk.
To deal with InputAttachment variation, ContextVk makes a copy of
mWriteDescriptors when program is bound, and then update inputAttachment
with framebuffer information. This not only removes unnecessary update
of mWriteDescriptors, but also removes the requirement that
mShaderBuffersDescriptorDesc has to reset and rebuild as a whole
(because mWriteDescriptors keeps mCurrentInfoIndex which gets
incremented as we build). This allows us to do further optimization in
future to do piece meal update of mDescriptorInfos with only the changed
data.

Bug: b/282194402
Change-Id: I443c7c3b85b7a2e2e93c68d40ea102533c43f76a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4540280
Reviewed-by: Roman Lavrov <romanl@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>