swift234

IRGen: Fix calling convention for functions with multiple indirect returns

The recent change to destructure tuples in SIL function return types
introduced some runtime changes where it was assumed that a SIL
function type like

$@convention(thin) () -> (@out X, @out Y)

Would have the same calling convention as the following C function:

void foo(void *X, void *Y);

Unfortunately, this only worked on x86-64, because the first @out
parameter in a SIL function type was lowered with the LLVM
'sret' attribute, which on i386 and ARM64 is not the same as the
first parameter to a function.

On i386, this manifested as a crash in a variety of executable tests
with a misaligned stack; on ARM64, a similar crash would occur because
the return value was initialized through the wrong register.

Hack around this by simply disabling 'sret' if a SIL function type
has multiple indirect return values.

Fixes <rdar://problem/24727411>.