Hide `CodegenCppVisitor::setup`. (#1550) #3280

Job	Run time
{ "os": "ubuntu-20.04" }	2m 29s
{ "flag_warnings": "ON", "glibc_asserts": "ON", "os": "ubuntu-22.04" }	3m 15s
{ "os": "macos-13" }	4m 56s
{ "os": "macos-14" }	5m 40s
{ "os": "windows-latest" }	12m 26s
{ "os": "ubuntu-22.04", "sanitizer": "address-leak" }	3m 32s
{ "os": "ubuntu-22.04", "enable_usecases": "On", "update_references": "On" }	4m 39s
{ "flag_warnings": "ON", "os": "ubuntu-22.04", "sanitizer": "undefined" }	3m 9s
commit-references	5s
	40m 11s

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