tools/clang/codesearch: migrate dyn_cast to dyn_cast_if_present #19152

Job	Run time
race	1m 54s
aux	1m 42s
race_dashboard	1m 56s
gvisor	1m 41s
old	1m 46s
dashboard	1m 56s
build	1m 56s
arch (presubmit_arch_linux)	1m 44s
arch (presubmit_arch_windows)	1m 43s
arch (presubmit_arch_netbsd)	1m 43s
arch (presubmit_arch_openbsd)	1m 42s
arch (presubmit_arch_executor)	1m 42s
arch (presubmit_arch_freebsd)	1m 43s
arch (presubmit_arch_darwin)	1m 42s
	24m 50s

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