rustc: Fix some ThinLTO internalization

First the `addPreservedGUID` function forgot to take care of "alias" summaries.
I'm not 100% sure what this is but the current code now matches upstream. Next
the `computeDeadSymbols` return value wasn't actually being used, but it needed
to be used! Together these should...

Closes #45195

src/test/run-pass/thinlto/auxiliary/dylib.rs

@@ -0,0 +1,16 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z thinlto -C codegen-units=8
+
+#[inline]
+pub fn foo(b: u8) {
+    b.to_string();
+}

src/test/run-pass/thinlto/auxiliary/thin-lto-inlines-aux.rs

@@ -0,0 +1,17 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// no-prefer-dynamic
+
+#![crate_type = "rlib"]
+
+pub fn bar() -> u32 {
+    3
+}

src/test/run-pass/thinlto/dylib-works.rs

@@ -0,0 +1,18 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:dylib.rs
+// min-llvm-version 4.0
+
+extern crate dylib;
+
+fn main() {
+    dylib::foo(1);
+}

src/test/run-pass/thinlto/thin-lto-inlines.rs

@@ -0,0 +1,39 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z thinlto -C codegen-units=8 -O
+// min-llvm-version 4.0
+// ignore-emscripten
+
+// We want to assert here that ThinLTO will inline across codegen units. There's
+// not really a great way to do that in general so we sort of hack around it by
+// praying two functions go into separate codegen units and then assuming that
+// if inlining *doesn't* happen the first byte of the functions will differ.
+
+pub fn foo() -> u32 {
+    bar::bar()
+}
+
+mod bar {
+    pub fn bar() -> u32 {
+        3
+    }
+}
+
+fn main() {
+    println!("{} {}", foo(), bar::bar());
+
+    unsafe {
+        let foo = foo as usize as *const u8;
+        let bar = bar::bar as usize as *const u8;
+
+        assert_eq!(*foo, *bar);
+    }
+}

src/test/run-pass/thinlto/thin-lto-inlines2.rs

@@ -0,0 +1,38 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z thinlto -C codegen-units=8 -O -C lto
+// aux-build:thin-lto-inlines-aux.rs
+// min-llvm-version 4.0
+// no-prefer-dynamic
+// ignore-emscripten
+
+// We want to assert here that ThinLTO will inline across codegen units. There's
+// not really a great way to do that in general so we sort of hack around it by
+// praying two functions go into separate codegen units and then assuming that
+// if inlining *doesn't* happen the first byte of the functions will differ.
+
+extern crate thin_lto_inlines_aux as bar;
+
+pub fn foo() -> u32 {
+    bar::bar()
+}
+
+fn main() {
+    println!("{} {}", foo(), bar::bar());
+
+    unsafe {
+        let foo = foo as usize as *const u8;
+        let bar = bar::bar as usize as *const u8;
+
+        assert_eq!(*foo, *bar);
+    }
+}
+