[clang] [analyzer][Z3][NFCI] Simplify getExpr* functions by taking a RetTy reference (PR #180801)

[Balázs Benics via cfe-commits]

steakhal wrote:

Note that I didn't checked this, and it's also not checked in the CI due to the `// REQUIRES: z3` comments in the tests and no CI bots build with Z3 AFAIK.

But it should work™️.

https://github.com/llvm/llvm-project/pull/180801