Demote proto3 grammar. #375

Job	Run time
linux (g++-9, Debug)	35s
linux-clang-extra (-fno-rtti, Debug)	1m 41s
linux (g++-9, Release)	45s
linux-gcc-extra (-fno-rtti, Debug)	2m 15s
linux (g++-10, Debug)	40s
linux-clang-extra (-fno-rtti, Release)	2m 23s
linux (g++-10, Release)	50s
linux-clang-extra (-fno-exceptions, Debug)	10s
linux-gcc-extra (-fno-rtti, Release)	2m 39s
linux (g++-11, Debug)	5m 33s
linux-clang-extra (-fno-exceptions, Release)	8s
linux-gcc-extra (-fno-exceptions, Debug)	7s
linux-clang-extra (-fms-extensions, Debug)	6m 46s
linux (g++-11, Release)	6m 43s
linux-gcc-extra (-fno-exceptions, Release)	6s
linux-clang-extra (-fms-extensions, Release)	6m 44s
linux (g++-12, Debug)	5m 59s
linux (g++-12, Release)	7m 43s
linux (clang++-12, Debug)	4m 29s
linux-old (clang++-7, Debug)	5m 54s
linux-old (clang++-7, Release)	6m 10s
linux (clang++-12, Release)	8m 8s
linux-old (clang++-8, Debug)	4m 28s
linux (clang++-13, Debug)	6m 19s
linux-old (clang++-8, Release)	8m 18s
linux (clang++-13, Release)	6m 53s
linux-old (clang++-9, Debug)	5m 33s
linux (clang++-14, Debug)	4m 40s
linux-old (clang++-9, Release)	10m 11s
linux (clang++-14, Release)	6m 48s
linux-old (clang++-10, Debug)	4m 46s
linux-old (clang++-10, Release)	10m 29s
linux-old (clang++-11, Debug)	4m 56s
linux-old (clang++-11, Release)	8m 13s
	2h 38m 2s

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