[clang] [clang][analyzer] Simplify code of StreamChecker (NFC). (PR #79312)

[Balázs Kéri via cfe-commits]

balazske wrote:

For clarification, the `evalRWCommon` new function is not planned to be used in the same way as other `eval*` calls, the name may be misleading. This function contains a "generic algorithm" that is called from the other `eval*` calls, but not used directly in `CallDescriptionMap`. The old `eval*` calls and the `evalRWCommon` are not meant to be called from each other, except `evalRWCommon` from other `eval*` calls. I agree that using lambdas makes debugging more difficult and lambdas are not ideal for big parts of code and code formatting is not nice either.

https://github.com/llvm/llvm-project/pull/79312