Fix LLVM version detection for `-rc1` #13398

Job	Run time
x86_64-musl-test	35m 42s
x86_64-gnu-test (1.7.3)	34m 25s
x86_64-gnu-test (1.8.2)	34m 17s
check_format	1m 40s
x86_64-gnu-test (1.9.2)	34m 16s
x86_64-gnu-test (1.10.1)	33m 10s
x86_64-gnu-test (1.11.2)	33m 30s
x86_64-gnu-test (1.12.2)	33m 44s
x86_64-gnu-test (1.13.3)	34m 46s
x86_64-gnu-test-preview_mt	5m 55s
x86_64-gnu-test (1.14.1)	3m 47s
x86_64-gnu-test (1.15.0)	32m 59s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	33m 1s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	33m 55s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	33m 46s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	34m 54s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	34m 54s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	35m 28s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	34m 42s
	9h 18m 51s

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