Add CI env for C++ and fix errors and warnings when compiling as C++ #1377

Job	Run time
check-misc (head)	12s
steep-check (head)	15s
test-memory (head)	35s
test-ruby (head, master)	6m 35s
test-ruby (3.0, master)	6m 10s
test-windows (head)	1m 31s
ruby-versions / Generate Ruby versions	3s
test-cpp (head, g++)	13s
test (2.5)	29s
test-cpp (head, clang++)	16s
test (2.6)	28s
test (2.7)	25s
test (3.0)	13s
test (3.1)	16s
test (3.2)	17s
test (3.3)	11s
test (head)	12s
	18m 21s

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