add sret + noalias to the out pointer parameter

[thestinger]

This brings Rust in line with how clang handles return pointers.

Example:

pub fn bar() -> [uint, .. 8] {
    let a = [0, .. 8];
    a
}

Before:

; Function Attrs: nounwind uwtable
define void @_ZN3bar17ha4635c6f704bfa334v0.0E([8 x i64]* nocapture, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #1 {
"function top level":
  %a = alloca [8 x i64], align 8
  %2 = bitcast [8 x i64]* %a to i8*
  call void @llvm.memset.p0i8.i64(i8* %2, i8 0, i64 64, i32 8, i1 false)
  %3 = bitcast [8 x i64]* %0 to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 64, i32 8, i1 false)
  ret void
}

After:

; Function Attrs: nounwind uwtable
define void @_ZN3bar17ha4635c6f704bfa334v0.0E([8 x i64]* noalias nocapture sret, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #1 {
"function top level":
  %2 = bitcast [8 x i64]* %0 to i8*
  call void @llvm.memset.p0i8.i64(i8* %2, i8 0, i64 64, i32 8, i1 false)
  ret void
}

Closes #9072
Closes #7298

[thestinger]

Not sure why an nopt build would have failed.... hopefully just a spurious failure from something else.

[thestinger]

This needs to happen on the declaration of functions, not while defining them.