[libc][hdrgen] Improve support for function-types as function args. (#227088)

Previously entries like
```
type: int (*)(const struct type_b **, const struct type_c **)
```
ended up assuming the type should be `struct_type_b_struct_type_c`.

Fix this by splitting the type strings into components, where each
component can be a type name itself (which is the case for a complex
function types like in the example above. This was raised during the
discussion in https://github.com/llvm/llvm-project/pull/223198

GitOrigin-RevId: e6e7da35405a3f6d91bc6a91d8bb2bd533a94276