MS ABI: Use the proper type for inalloca args

Summary:
The definition of a type later in a translation unit may change it's
type from {}* to (%struct.foo*)*.  Earlier function definitions may use
the former while more recent definitions might use the later.  This is
fine until they interact with one another (like one calling the other).
In these cases, a bitcast is needed because the inalloca must match the
function call but the store to the lvalue which initializes the argument
slot has to match the rvalue's type.

This technique is along the same lines with what the other,
non-inalloca, codepaths perform.

This fixes PR19287.

Reviewers: rnk

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3224