4cb9d214345550cb0299139b2badb73ba1e53532 blockstorage: add LIFETIMEBOUND to GetFirstStoredBlock()::start_time (Jon Atack)
Pull request description:
Suggested in https://github.com/bitcoin/bitcoin/pull/25016#discussion_r862330288, the lifetimebound attribute here indicates that a resource owned by the `start_block` param of `CBlockIndex* BlockManager::GetFirstStoredBlock()` can be retained by the method's return value, which enables detecting the use of out-of-scope stack memory (ASan `stack-use-after-scope`) at compile time.
See https://releases.llvm.org/12.0.0/tools/clang/docs/AttributeReference.html#lifetimebound and #22278 for related discussion, and #25040 for a similar example.
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Tree-SHA512: a3f5ef83ebb6f08555d7c89f2437a682071b4ad77a7aa3326b6d2282c909bf9fcf4dac6bf05ee1d9931f2102cad4a02df5468bde1cf377d7126e84e8541604dc
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