auto merge of #14819 : michaelwoerister/rust/unique_type_id, r=alexcrichton

With this change, rustc creates a unique type identifier for types in debuginfo. These type identifiers are used by LLVM to correctly handle link-time-optimization scenarios but also help rustc with dealing with inlining from other crates. For more information, see the documentation block at the top of librustc/middle/trans/debuginfo.rs and also [my blog post about the topic](http://michaelwoerister.github.io/2014/06/05/rust-debuginfo-and-unique-type-identifiers.html). This should fix the LTO issues that have been popping up lately. 

The changes to the debuginfo module's inner workings are also improved by this. Metadata uniquing of pointer types is not handled explicitly instead of relying on LLVM doing the right thing behind the scenes, and region parameters on types should not lead to metadata duplication anymore.

There are two things that I'd like to get some feedback on:
1. IDs for named items consist of two parts: The [Strict Version Hash](https://github.com/mozilla/rust/blob/0.10/src/librustc/back/svh.rs#L11) of their defining crate and the AST node id of their definition within that crate. My question is: Is the SVH a good choice for identifying the crate? Is it even going to stay? The [crate-id RFC](https://github.com/rust-lang/rfcs/pull/109) got me confused.
2. Unique Type Identifiers can be arbitrary strings and right now the format is rather verbose. For debugging this is nice, because one can infer a lot about a type from the type id alone (it's more or less a signature). For deeply nested generics, id strings could get rather long though. One option to limit the id size would be to use some hashcode instead of the full id (anything that avoids collision as much as possible). Another option would be to use a more compact representation, like ty_encode. This reduces size but also readability.
Since these ID's only show up in LLVM IR, I'm inclined to just leave in the verbose format for now, and only act if sizes of rlibs become a problem.

src/librustc/lib/llvm.rs

@@ -1809,6 +1809,7 @@ pub mod llvm {
         pub fn LLVMRustDestroyArchive(AR: ArchiveRef);
 
         pub fn LLVMRustSetDLLExportStorageClass(V: ValueRef);
+        pub fn LLVMVersionMajor() -> c_int;
         pub fn LLVMVersionMinor() -> c_int;
 
         pub fn LLVMRustGetSectionName(SI: SectionIteratorRef,

src/rustllvm/RustWrapper.cpp

@@ -318,7 +318,7 @@ extern "C" LLVMValueRef LLVMDIBuilderCreateStructType(
         unwrapDI<DIArray>(Elements),
         RunTimeLang,
         unwrapDI<DIType>(VTableHolder)
-#if LLVM_VERSION_MINOR >= 5
+#if LLVM_VERSION_MINOR >= 4
         ,UniqueId
 #endif
         ));
@@ -510,7 +510,7 @@ extern "C" LLVMValueRef LLVMDIBuilderCreateUnionType(
         Flags,
         unwrapDI<DIArray>(Elements),
         RunTimeLang
-#if LLVM_VERSION_MINOR >= 5
+#if LLVM_VERSION_MINOR >= 4
         ,UniqueId
 #endif
         ));
@@ -734,6 +734,11 @@ LLVMVersionMinor() {
     return LLVM_VERSION_MINOR;
 }
 
+extern "C" int
+LLVMVersionMajor() {
+    return LLVM_VERSION_MAJOR;
+}
+
 // Note that the two following functions look quite similar to the
 // LLVMGetSectionName function. Sadly, it appears that this function only
 // returns a char* pointer, which isn't guaranteed to be null-terminated. The

src/test/auxiliary/cross_crate_debuginfo_type_uniquing.rs

@@ -0,0 +1,26 @@
+// Copyright 2013-2014 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"]
+// compile-flags:-g
+
+struct S1;
+
+impl S1 {
+    fn f(&mut self) { }
+}
+
+
+struct S2;
+
+impl S2 {
+    fn f(&mut self) { }
+}

src/test/debuginfo/cross-crate-type-uniquing.rs

@@ -0,0 +1,24 @@
+// Copyright 2013-2014 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.
+
+// ignore-android: FIXME(#10381)
+
+// aux-build:cross_crate_debuginfo_type_uniquing.rs
+extern crate cross_crate_debuginfo_type_uniquing;
+
+// no-prefer-dynamic
+// compile-flags:-g -Zlto
+
+pub struct C;
+pub fn p() -> C {
+    C
+}
+
+fn main() { }