tools/clang/codesearch: migrate dyn_cast to dyn_cast_if_present #19155

Job	Run time
race_dashboard	3m 18s
gvisor	4m 3s
race	7m 51s
build	4m 57s
old	7m 11s
aux	3m 52s
arch (presubmit_arch_windows)	3m 36s
arch (presubmit_arch_netbsd)	3m 55s
arch (presubmit_arch_freebsd)	6m 33s
arch (presubmit_arch_linux)	10m 43s
dashboard	2m 44s
arch (presubmit_arch_openbsd)	3m 58s
arch (presubmit_arch_executor)	3m 17s
arch (presubmit_arch_darwin)	3m 48s
	1h 9m 46s

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