[MSP430] Fix for byval parameters

[llvmbot]

Bugzilla Link	14096
Version	trunk
OS	Linux
Attachments	Patch, Patch including testcase
Reporter	LLVM Bugzilla Contributor
CC	@asl

Extended Description

I noticed that the 'byval' parameter attribute is ignored in the MSP430
backend. This causes functions that take a struct as argument to miscompile.
Specifically, structs that should be passed by value are passed as a pointer
instead.

I have attached a simple patch that addresses this problem.