tools/syz-declextract/clangtool: fix getBitWidthValue for LLVM>=21 #16127

Job	Run time
aux	3m 55s
build	3m 21s
arch (presubmit_arch_freebsd)	6m 11s
dashboard	2m 16s
arch (presubmit_arch_linux)	10m 21s
arch (presubmit_arch_netbsd)	3m 47s
arch (presubmit_arch_windows)	3m 30s
arch (presubmit_arch_darwin)	3m 40s
arch (presubmit_arch_openbsd)	3m 46s
race_dashboard	2m 59s
race	9m 31s
old	6m 49s
arch (presubmit_arch_executor)	3m 21s
gvisor	3m 28s
	1h 6m 55s

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