Auto merge of #39747 - mattico:fix-llvm4-createcompileunit, r=alexcrichton

[LLVM 4.0] Fix CreateCompileUnit

This is largely identical to @dylanmckay's [patch](https://github.com/dylanmckay), except that it doesn't try to use `file_metadata()`. I don't think that is necessary because we don't want the compile unit to be added to  `debug_context.created_files`, though I'd like confirmation from someone who knows for sure. If that is needed, I can modify `file_metadata_()` so that it can be used from `compile_unit_metadata()`.

Author: bors

src/librustc_llvm/ffi.rs

@@ -1334,8 +1334,7 @@ extern "C" {
 
     pub fn LLVMRustDIBuilderCreateCompileUnit(Builder: DIBuilderRef,
                                               Lang: c_uint,
-                                              File: *const c_char,
-                                              Dir: *const c_char,
+                                              File: DIFile,
                                               Producer: *const c_char,
                                               isOptimized: bool,
                                               Flags: *const c_char,

src/librustc_trans/debuginfo/metadata.rs

@@ -791,12 +791,15 @@ pub fn compile_unit_metadata(scc: &SharedCrateContext,
     let producer = CString::new(producer).unwrap();
     let flags = "\0";
     let split_name = "\0";
-    return unsafe {
-        llvm::LLVMRustDIBuilderCreateCompileUnit(
+
+    unsafe {
+        let file_metadata = llvm::LLVMRustDIBuilderCreateFile(
+            debug_context.builder, compile_unit_name, work_dir.as_ptr());
+
+        return llvm::LLVMRustDIBuilderCreateCompileUnit(
             debug_context.builder,
             DW_LANG_RUST,
-            compile_unit_name,
-            work_dir.as_ptr(),
+            file_metadata,
             producer.as_ptr(),
             sess.opts.optimize != config::OptLevel::No,
             flags.as_ptr() as *const _,

src/rustllvm/RustWrapper.cpp

@@ -474,11 +474,19 @@ extern "C" void LLVMRustDIBuilderFinalize(LLVMRustDIBuilderRef Builder) {
 }
 
 extern "C" LLVMRustMetadataRef LLVMRustDIBuilderCreateCompileUnit(
-    LLVMRustDIBuilderRef Builder, unsigned Lang, const char *File,
-    const char *Dir, const char *Producer, bool isOptimized, const char *Flags,
+    LLVMRustDIBuilderRef Builder, unsigned Lang, LLVMRustMetadataRef FileRef,
+    const char *Producer, bool isOptimized, const char *Flags,
     unsigned RuntimeVer, const char *SplitName) {
-  return wrap(Builder->createCompileUnit(Lang, File, Dir, Producer, isOptimized,
+  auto *File = unwrapDI<DIFile>(FileRef);
+
+#if LLVM_VERSION_GE(4, 0)
+  return wrap(Builder->createCompileUnit(Lang, File, Producer, isOptimized,
                                          Flags, RuntimeVer, SplitName));
+#else
+  return wrap(Builder->createCompileUnit(Lang, File->getFilename(),
+      File->getDirectory(), Producer, isOptimized,
+      Flags, RuntimeVer, SplitName));
+#endif
 }
 
 extern "C" LLVMRustMetadataRef