rust_bindgen should pass opt-level flags to clang

**Description**
Currently, when running `clang`, `rust_bindgen` never passes a `-O` flag. In optimized builds (`-c opt`), it should pass the appropriate `-O` flag to match the C toolchain.

**Reproduction steps**
`bazel build -c opt` a `rust_bindgen` target and observe that the flags passed to clang do not include `-O`

**Impact**
The impact of this is minor, as the output is not a compiled C artifact but a Rust library, but in pathological cases the difference in optimization level could affect the output. Additionally, this has been found to lead to warnings around [`_FORTIFY_SOURCE`](https://developers.redhat.com/articles/2023/07/04/developers-guide-secure-coding-fortifysource) (`warning: _FORTIFY_SOURCE requires compiling with optimization (-O)`), though this is likely because of a mix of GCC and Clang headers being used 🙈.

**Bazel and rules_rust version**
```
bazel 9.1.0
bazel_dep(name = "rules_rust_bindgen", version = "0.74.0")
```

Start by locating the rust_bindgen code that assembles and passes flags to clang, then reproduce the issue with a rust_bindgen target using `bazel build -c opt`. Done means the clang invocation includes the optimization flag corresponding to the optimized C toolchain configuration.

Written by the indexing model from the issue text.