[Mangler] Calculate the argument list byte count suffix correctly when returning large values

`__stdcall`, `__fastcall` and `__vectorcall` return large values via a
hidden pointer argument. However, the size of that argument should not
be included in the argument list byte count suffix added to the
function's decorated name.

This patch fixes that issue so that LLVM generates the same decorated
name as MSVC does.

MSVC example: https://godbolt.org/z/nc35MKPhr

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D110719