result location mechanism for struct initialization

```zig
export fn entry() void {
    const static = Foo{
        .x = 9,
        .bar = Bar{ .y = 10 },
    };
    const runtime = foo(true);
}

fn foo(c: bool) Foo {
    return Foo{
        .x = 12,
        .bar = if (c) bar1() else bar2(),
    };
}

fn bar1() Bar {
    return Bar{ .y = 34 };
}

fn bar2() Bar {
    return Bar{ .y = 56 };
}
```

```llvm
@0 = internal unnamed_addr constant %Foo { i32 9, %Bar { i32 10 } }, align 4
@1 = internal unnamed_addr constant %Bar { i32 34 }, align 4
@2 = internal unnamed_addr constant %Bar { i32 56 }, align 4

define void @entry() #2 !dbg !35 {
Entry:
  %runtime = alloca %Foo, align 4
  call void @llvm.dbg.declare(metadata %Foo* @0, metadata !39, metadata !DIExpression()), !dbg !50
  call fastcc void @foo(%Foo* sret %runtime, i1 true), !dbg !51
  call void @llvm.dbg.declare(metadata %Foo* %runtime, metadata !49, metadata !DIExpression()), !dbg !52
  ret void, !dbg !53
}

define internal fastcc void @foo(%Foo* nonnull sret, i1) unnamed_addr #2 !dbg !54 {
Entry:
  %c = alloca i1, align 1
  store i1 %1, i1* %c, align 1
  call void @llvm.dbg.declare(metadata i1* %c, metadata !60, metadata !DIExpression()), !dbg !61
  %2 = getelementptr inbounds %Foo, %Foo* %0, i32 0, i32 0, !dbg !62
  store i32 12, i32* %2, align 4, !dbg !62
  %3 = getelementptr inbounds %Foo, %Foo* %0, i32 0, i32 1, !dbg !64
  %4 = load i1, i1* %c, align 1, !dbg !65
  br i1 %4, label %Then, label %Else, !dbg !65

Then:                                             ; preds = %Entry
  call fastcc void @bar1(%Bar* sret %3), !dbg !66
  br label %EndIf, !dbg !64

Else:                                             ; preds = %Entry
  call fastcc void @bar2(%Bar* sret %3), !dbg !67
  br label %EndIf, !dbg !64

EndIf:                                            ; preds = %Else, %Then
  ret void, !dbg !68
}

define internal fastcc void @bar1(%Bar* nonnull sret) unnamed_addr #2 !dbg !69 {
Entry:
  %1 = bitcast %Bar* %0 to i8*, !dbg !73
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %1, i8* align 4 bitcast (%Bar* @1 to i8*), i64 4, i1 false), !dbg !73
  ret void, !dbg !73
}

define internal fastcc void @bar2(%Bar* nonnull sret) unnamed_addr #2 !dbg !75 {
Entry:
  %1 = bitcast %Bar* %0 to i8*, !dbg !76
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %1, i8* align 4 bitcast (%Bar* @2 to i8*), i64 4, i1 false), !dbg !76
  ret void, !dbg !76
}

!39 = !DILocalVariable(name: "static", scope: !40, file: !5, line: 2, type: !41)
!49 = !DILocalVariable(name: "runtime", scope: !40, file: !5, line: 6, type: !41)
```

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