Symbols that these extensions introduced were only accepted if the
extension directive was present.  This is not necessary for es 320
shaders.  This change adds a new tag to builtin variable and function
declarations in the translator, namely "essl_extension_becomes_core_in"
which makes gen_builtin_symbols.py automatically create two entries for
the builtin; one with the specified level and extension, one with the
core level and without extension.  Entries in
builtin_function_declarations.txt can potentially be deduplicated as a
follow up to take advantage of this tag, though likely an anologous
"glsl_extension_becomes_core_in" needs to be introduced.

Bug: angleproject:5557
Bug: angleproject:5579
Change-Id: I84c19f48a3ccc89d82d80a4f35f7833205bbc88f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2649449Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>