Ruby

Add Clang compilation CI job on Ubuntu #8047

Job	Run time
C99_compile (4.0)	1m 1s
clang_compile (head)	1m 26s
C99_compile (head)	50s
clang_compile (4.0)	1m 29s
test (head, stdlib_test rubocop)	1m 56s
test (3.2, test)	2m 3s
test (4.0, stdlib_test, --enable-frozen-string-literal)	2m 1s
test (4.0, test, --enable-frozen-string-literal)	2m 11s
test (4.0, test)	2m 34s
test (head, test)	2m 5s
test (3.3, test)	2m 49s
test (4.0, stdlib_test)	1m 53s
test (3.4, test)	2m 29s
test (4.0, typecheck_test)	1m 27s
test (asan-release, test)	8m 6s
test (4.0, rubocop validate test_doc build test_generate_stdlib raap)	2m 23s
	36m 43s

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