Ruby

Add Clang compilation CI job on Ubuntu #8048

Job	Run time
C99_compile (head)	52s
clang_compile (head)	1m 24s
clang_compile (4.0)	1m 26s
C99_compile (4.0)	1m 12s
test (4.0, rubocop validate test_doc build test_generate_stdlib raap)	2m 25s
test (head, test)	2m 16s
test (3.2, test)	2m 28s
test (4.0, typecheck_test)	1m 12s
test (4.0, stdlib_test, --enable-frozen-string-literal)	1m 49s
test (asan-release, test)	8m 1s
test (3.3, test)	2m 49s
test (head, stdlib_test rubocop)	1m 54s
test (4.0, test)	2m 29s
test (4.0, stdlib_test)	1m 54s
test (4.0, test, --enable-frozen-string-literal)	2m 26s
test (3.4, test)	2m 16s
	36m 53s

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