Add tests for raw-dylib with vectorcall, and fix vectorcall code generation
* Adds tests for using `raw-dylib` (#58713) with `vectorcall`.
* Fixed code generation for `vectorcall` (parameters have to be marked with `InReg`, just like `fastcall`).
* Enabled running the `raw-dylib` `fastcall` tests when using MSVC (since I had to add support in the test for running MSVC-only tests since GCC doesn't support `vectorcall`).
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