(`c2rust-analyze`) Handle string literals

[kkysen]

Currently, c2rust-analyze can't handle any string literals, such as "" or b"":

callgraph traversal order:
  DefId(0:3 ~ string_casts[c5c4]::f)
visit_statement(StorageLive(_1))
visit_statement(_1 = const "")
thread 'rustc' panicked at 'unexpected pointer type in &str', c2rust-analyze/src/context.rs:506:9
stack backtrace:
   0: rust_begin_unwind
             at /rustc/d394408fb38c4de61f765a3ed5189d2731a1da91/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/d394408fb38c4de61f765a3ed5189d2731a1da91/library/core/src/panicking.rs:142:14
   2: c2rust_analyze::context::label_no_pointers::{{closure}}
             at ./src/context.rs:503:9
   3: c2rust_analyze::labeled_ty::LabeledTyCtxt<L>::label
             at ./src/labeled_ty.rs:157:21
   4: c2rust_analyze::context::label_no_pointers
             at ./src/context.rs:502:5
   5: <rustc_middle::mir::syntax::Operand as c2rust_analyze::context::TypeOf>::type_of
             at ./src/context.rs:494:41
   6: <&T as c2rust_analyze::context::TypeOf>::type_of
             at ./src/context.rs:464:9
   7: c2rust_analyze::context::AnalysisCtxt::type_of
             at ./src/context.rs:241:9
   8: c2rust_analyze::context::AnalysisCtxt::type_of_rvalue
             at ./src/context.rs:304:36
   9: c2rust_analyze::dataflow::type_check::TypeChecker::visit_statement
             at ./src/dataflow/type_check.rs:233:30
  10: c2rust_analyze::dataflow::type_check::visit
             at ./src/dataflow/type_check.rs:388:13
  11: c2rust_analyze::dataflow::generate_constraints
             at ./src/dataflow/mod.rs:326:5
  12: c2rust_analyze::run
             at ./src/main.rs:480:45
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Fixing this is a prerequisite to #833.