In GenCall, fix the IR gen for C++ method calls as under MSVC as the
calling conventions for free functions and C++ methods can be
different. This also fixes the missing inreg (on sret arguments)
issues on Windows ARM64. Also refactor to use CGFunctionInfo
returnInfo isSretAfterThis to detect when to reorder the sret and the
this arguments under MSVC.
In ClagImporter, don't drop the return type for the compound
assignment operators such as operator+= when the return value is a
reference so that the CGFunctionInfo will be correctly indicate an
indirect return for the compound assignment operators.
