Support `import_name` and `import_module` attributes when targeting `wasm32`

I know we already have wasm_import_module_name, but clang already has attributes for that! Supporting them directly would be much more convenient, I think?

See: https://clang.llvm.org/docs/AttributeReference.html#import-module

Another big motivation for this would be that it allows importing from different modules in the same header.

Input C/C++ Header

#ifdef __wasm__
__attribute__((visibility("default"), import_module("foo"), import_name("bar")))
#endif
void foo_bar();

Bindgen Invocation

$ bindgen input.h -- -target wasm32

Note that clang must be passed -target wasm32 to recognize the attributes.

Actual Results

/* automatically generated by rust-bindgen 0.70.1 */

extern "C" {
    pub fn foo_bar();
}

Expected Results

/* automatically generated by rust-bindgen 0.70.1 */

#[link(wasm_import_module = "foo")]
extern "C" {
    #[link_name = "bar"]
    pub fn foo_bar();
}

Start by tracing the existing wasm_import_module_name handling and how bindgen processes Clang attributes when invoked with -- -target wasm32. Use the provided input.h example as the reproduction, and consider the work complete when generated bindings include #[link(wasm_import_module = "foo")] and #[link_name = "bar"] for the annotated function.