A new AST validation is added to ensure that the same TStructure or
TInterfaceBlock is not redundantly defined.  This helps with SPIR-V
generation by allowing the id to be used as key in a hash map that looks
up the corresponding SPIR-V type id.

A bug is fixed where the Vulkan driver uniform declaration created two
identical declarations for ANGLEDepthRangeParams.

A number of other bugs are also fixed in this change, where if a
variable declaration is eliminated (for example due to constant folding,
or inactive interface variable removal) and it contained a struct
specifier, the struct declaration was also removed.  OutputGLSLBase had
a hack where structs were declared on first encounter, which was
incorrect as the scope of the declaration could change.  Those bugs are
fixed and this hack is removed.

Bug: angleproject:2733
Bug: angleproject:4889
Bug: angleproject:5936
Change-Id: I8e13748c0bf552ae8b052249282769a1f0775603
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2881942Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>