EOpCallBuiltInFunction is removed in this change, as well as the
"op": "auto" property in builtin_function_declarations.txt.  Instead,
gen_builtin_symbols.py automatically generates Ops for every built-in
function and generates the TOperator enum accordingly.

This simplifies SPIR-V code generation by allowing switches to be used
on operators instead of string comparisons.

Bug: angleproject:4589
Bug: angleproject:4889
Change-Id: Ia351524400b0e12a10a5572e27e9b88c6ec2e61c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2958869
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>