Since line raster emulation was changed to use specialization constants,
it has been possible to compile shaders at link time.  However, program
pipeline objects would have required keeping the shader sources around
for recompilation.  Now that all necessary decorations are modified
directly in SPIR-V, it's possible to compile the shaders at link time
and forget about their sources.  Program pipeline objects then simply
"reconfigure" the generated SPIR-V.

A next step could be to also create the Vulkan pipeline object at link
time.  A number of failures due to gaps in CTS testing prevent that work
currently.  In particular, in some situations the generated SPIR-V is
not per spec, for example it may contain vertex attributes with aliasing
locations, or have transform feedback capture of array elements
misconfigured.

Bug: angleproject:3394
Bug: angleproject:4253
Change-Id: I54c0884cf056b511a4a306225cc6ed2cef84d257
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2023186
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>