Translator can accept gl_LastFragData and 'inout' variable to gain
access to framebuffer attachment data. The Vulkan translator replaces
it with the SubpassInput type variable. Note that this works only for
the noncoherent version of the extension.

Bug: angleproject:5454
Test: *EXTShaderFramebufferFetchNoncoherent*.*
Change-Id: I392f84ee3ad3eb9fbd09d0b7ff83731a9a3f33f6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2598060Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>