Refactor toolchain/check/testdata/interop/cpp/function_decl.carbon (carbon-language#5457)

* Split to 4 files: `function_param_int16`, `function_param_int32`,
`function_param_unsupported.carbon`, `function_return`.
* Add // === headlines on group of file shards.
* Rename shard files to shorten them and make them more consistent given
the file context.
* Deduplicate identical .h files and group their tests.

Potential future improvements:
* Split further. For example, pointers and references might be somewhat
separate from int primitives.
* Remove `import_` shard file prefix, as it repeats itself, but leaving
for now as it makes it more explicit.

Part of carbon-language#5263

Author: bricknerb