Let LLVM 5 add DW_OP_deref to indirect args itself

We needed to manually added the `DW_OP_deref` ourselves in earlier LLVM,
but starting with [D31439] in LLVM 5, it appears that LLVM will always
handle this itself.  When we were still adding this manually, the
resulting `.debug_loc` had too many derefs, and this failed test
`debuginfo/by-value-self-argument-in-trait-impl.rs`.

[D31439]: https://reviews.llvm.org/D31439

Fixes #47611.
cc @alexcrichton
r? @michaelwoerister

src/librustc_trans/mir/mod.rs

@@ -487,16 +487,18 @@ fn arg_local_refs<'a, 'tcx>(bx: &Builder<'a, 'tcx>,
                 // The Rust ABI passes indirect variables using a pointer and a manual copy, so we
                 // need to insert a deref here, but the C ABI uses a pointer and a copy using the
                 // byval attribute, for which LLVM does the deref itself, so we must not add it.
+                // Starting with D31439 in LLVM 5, it *always* does the deref itself.
                 let mut variable_access = VariableAccess::DirectVariable {
                     alloca: place.llval
                 };
-
-                if let PassMode::Indirect(ref attrs) = arg.mode {
-                    if !attrs.contains(ArgAttribute::ByVal) {
-                        variable_access = VariableAccess::IndirectVariable {
-                            alloca: place.llval,
-                            address_operations: &deref_op,
-                        };
+                if unsafe { llvm::LLVMRustVersionMajor() < 5 } {
+                    if let PassMode::Indirect(ref attrs) = arg.mode {
+                        if !attrs.contains(ArgAttribute::ByVal) {
+                            variable_access = VariableAccess::IndirectVariable {
+                                alloca: place.llval,
+                                address_operations: &deref_op,
+                            };
+                        }
                     }
                 }