Enables OES/EXT_shader_io_blocks extensions in Vulkan backends.

With shader I/O blocks, the varyings can now be an array of struct (the
block itself) of struct (nested in the block).

This change is missing a number of features.  In particular, if the
shader I/O block has a location decoration in the middle of the block,
that is not handled yet.

Based on changes from m.maiya@samsung.com and jmadill@chromium.org.

Bug: angleproject:3580
Tests: dEQP-GLES31.functional.shaders.linkage.es31.io_block.*
 dEQP-GLES31.functional.separate_shader.validation.es31.io_blocks.*
 dEQP-GLES31.functional.program_interface_query.program_input.*
 dEQP-GLES31.functional.program_interface_query.program_output.*
Change-Id: I593840475d2365ff6c9ce7b2290f5ee462a30dfb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2567645Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>