Declare by-value on-stack parameters to be noalias

Function parameters that are to be passed by value but don't fit into a
single register are currently passed by creating a copy on the stack and
passing a pointer to that copy to the callee. Since the copy is made
just for the function call, there are no aliases.

For example, this sometimes allows LLVM to eliminate unnecessary calls
to drop glue. Given

````rust
struct Foo {
    a: int,
    b: Option<~str>,
}

extern {
    fn eat(eat: Option<~str>);
}

pub fn foo(v: Foo) {
    match v {
        Foo { a: _, b } => unsafe { eat(b) }
    }
}
````

LLVM currently can't eliminate the drop call for the string, because it
only sees a _pointer_ to Foo, for which it has to expect an alias. So we
get:

````llvm
; Function Attrs: uwtable
define void @_ZN3foo20h9f32c90ae7201edbxaa4v0.0E(%struct.Foo* nocapture) unnamed_addr #0 {
"_ZN34std..option..Option$LT$$UP$str$GT$9glue_drop17hc39b3015f3b9c69dE.exit":
  %1 = getelementptr inbounds %struct.Foo* %0, i64 0, i32 1, i32 0
  %2 = load { i64, i64, [0 x i8] }** %1, align 8
  store { i64, i64, [0 x i8] }* null, { i64, i64, [0 x i8] }** %1, align 8
  %3 = ptrtoint { i64, i64, [0 x i8] }* %2 to i64
  %.fca.0.insert = insertvalue { i64 } undef, i64 %3, 0
  tail call void @eat({ i64 } %.fca.0.insert)
  %4 = load { i64, i64, [0 x i8] }** %1, align 8
  %5 = icmp eq { i64, i64, [0 x i8] }* %4, null
  br i1 %5, label %_ZN3Foo9glue_drop17hf611996539d3036fE.exit, label %"_ZN8_$UP$str9glue_drop17h15dbdbe2b8897a98E.exit.i.i"

"_ZN8_$UP$str9glue_drop17h15dbdbe2b8897a98E.exit.i.i": ; preds = %"_ZN34std..option..Option$LT$$UP$str$GT$9glue_drop17hc39b3015f3b9c69dE.exit"
  %6 = bitcast { i64, i64, [0 x i8] }* %4 to i8*
  tail call void @free(i8* %6) #1
  br label %_ZN3Foo9glue_drop17hf611996539d3036fE.exit

_ZN3Foo9glue_drop17hf611996539d3036fE.exit:       ; preds = %"_ZN34std..option..Option$LT$$UP$str$GT$9glue_drop17hc39b3015f3b9c69dE.exit", %"_ZN8_$UP$str9glue_drop17h15dbdbe2b8897a98E.exit.i.i"
  ret void
}
````

But with the `noalias` attribute, it can safely optimize that to:

````llvm
define void @_ZN3foo20hd28431f929f0d6c4xaa4v0.0E(%struct.Foo* noalias nocapture) unnamed_addr #0 {
_ZN3Foo9glue_drop17he9afbc09d4e9c851E.exit:
  %1 = getelementptr inbounds %struct.Foo* %0, i64 0, i32 1, i32 0
  %2 = load { i64, i64, [0 x i8] }** %1, align 8
  store { i64, i64, [0 x i8] }* null, { i64, i64, [0 x i8] }** %1, align 8
  %3 = ptrtoint { i64, i64, [0 x i8] }* %2 to i64
  %.fca.0.insert = insertvalue { i64 } undef, i64 %3, 0
  tail call void @eat({ i64 } %.fca.0.insert)
  ret void
}
````
auto merge of #12296 : dotdash/rust/byval_noalias, r=cmr

Function parameters that are to be passed by value but don't fit into a
single register are currently passed by creating a copy on the stack and
passing a pointer to that copy to the callee. Since the copy is made
just for the function call, there are no aliases.

For example, this sometimes allows LLVM to eliminate unnecessary calls
to drop glue. Given

````rust
struct Foo {
    a: int,
    b: Option<~str>,
}

extern {
    fn eat(eat: Option<~str>);
}

pub fn foo(v: Foo) {
    match v {
        Foo { a: _, b } => unsafe { eat(b) }
    }
}
````

LLVM currently can't eliminate the drop call for the string, because it
only sees a _pointer_ to Foo, for which it has to expect an alias. So we
get:

````llvm
; Function Attrs: uwtable
define void @_ZN3foo20h9f32c90ae7201edbxaa4v0.0E(%struct.Foo* nocapture) unnamed_addr #0 {
"_ZN34std..option..Option$LT$$UP$str$GT$9glue_drop17hc39b3015f3b9c69dE.exit":
  %1 = getelementptr inbounds %struct.Foo* %0, i64 0, i32 1, i32 0
  %2 = load { i64, i64, [0 x i8] }** %1, align 8
  store { i64, i64, [0 x i8] }* null, { i64, i64, [0 x i8] }** %1, align 8
  %3 = ptrtoint { i64, i64, [0 x i8] }* %2 to i64
  %.fca.0.insert = insertvalue { i64 } undef, i64 %3, 0
  tail call void @eat({ i64 } %.fca.0.insert)
  %4 = load { i64, i64, [0 x i8] }** %1, align 8
  %5 = icmp eq { i64, i64, [0 x i8] }* %4, null
  br i1 %5, label %_ZN3Foo9glue_drop17hf611996539d3036fE.exit, label %"_ZN8_$UP$str9glue_drop17h15dbdbe2b8897a98E.exit.i.i"

"_ZN8_$UP$str9glue_drop17h15dbdbe2b8897a98E.exit.i.i": ; preds = %"_ZN34std..option..Option$LT$$UP$str$GT$9glue_drop17hc39b3015f3b9c69dE.exit"
  %6 = bitcast { i64, i64, [0 x i8] }* %4 to i8*
  tail call void @free(i8* %6) #1
  br label %_ZN3Foo9glue_drop17hf611996539d3036fE.exit

_ZN3Foo9glue_drop17hf611996539d3036fE.exit:       ; preds = %"_ZN34std..option..Option$LT$$UP$str$GT$9glue_drop17hc39b3015f3b9c69dE.exit", %"_ZN8_$UP$str9glue_drop17h15dbdbe2b8897a98E.exit.i.i"
  ret void
}
````

But with the `noalias` attribute, it can safely optimize that to:

````llvm
define void @_ZN3foo20hd28431f929f0d6c4xaa4v0.0E(%struct.Foo* noalias nocapture) unnamed_addr #0 {
_ZN3Foo9glue_drop17he9afbc09d4e9c851E.exit:
  %1 = getelementptr inbounds %struct.Foo* %0, i64 0, i32 1, i32 0
  %2 = load { i64, i64, [0 x i8] }** %1, align 8
  store { i64, i64, [0 x i8] }* null, { i64, i64, [0 x i8] }** %1, align 8
  %3 = ptrtoint { i64, i64, [0 x i8] }* %2 to i64
  %.fca.0.insert = insertvalue { i64 } undef, i64 %3, 0
  tail call void @eat({ i64 } %.fca.0.insert)
  ret void
}
````