[clang] Reset PostAllocaInsertPt when the AllocaInsertPt is changed in clang::CodeGenFunction (PR #98668)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Does this affect the generated IR in some existing case?  If it does, can you include a regression test in clang/test/CodeGen?

https://github.com/llvm/llvm-project/pull/98668