Ruby

Add Clang compilation CI job on Ubuntu #8042

Job	Run time
clang_compile (4.0)	1m 19s
clang_compile (head)	1m 14s
C99_compile (4.0)	49s
C99_compile (head)	1m 2s
test (3.2, test)	2m 26s
test (4.0, test, --enable-frozen-string-literal)	2m 26s
test (3.4, test)	2m 48s
test (3.3, test)	2m 36s
test (4.0, test)	2m 32s
test (head, test)	2m 11s
test (4.0, rubocop validate test_doc build test_generate_stdlib raap)	2m 29s
test (head, stdlib_test rubocop)	1m 55s
test (4.0, stdlib_test, --enable-frozen-string-literal)	1m 57s
test (asan-release, test)	8m 8s
test (4.0, typecheck_test)	1m 17s
test (4.0, stdlib_test)	2m 2s
	37m 11s

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