[clang] [clang][CodeGen][MSVC] Return vector types from methods indirectly (PR #157365)

[Reid Kleckner via cfe-commits]

rnk wrote:

> I worry that the real invariant here is that instance methods must always pass `this` in the first argument register (`RCX`), and we've only covered a few of the cases that enable indirect return. Like, I wonder if we can hit this via member pointers.

[I guess not](https://godbolt.org/z/so753WKse)

https://github.com/llvm/llvm-project/pull/157365