[clang] [Clang] Generate the GEP instead of adding AST nodes (PR #73730)

[Bill Wendling via cfe-commits]

bwendling wrote:

It's the __bdos cases that really benefit from the `ExprLValueMap`. The `LocalDeclMap` will have any `allocas` or globals for local C decls. From that, I'll be able to get enough information to generate the `count` lookup. The `ExprLValueMap` improves this by allowing us to use an expression with side-effects as long as it was already emitted.

https://github.com/llvm/llvm-project/pull/73730