Fix invalid DWARF for enums when using thinlto

We were setting the same identifier for both the DW_TAG_structure_type
and the DW_TAG_variant_part. This becomes a problem when using thinlto
becauses it uses the identifier as a key for a map of types that is used
to delete duplicates based on the ODR, so one of them is deleted as a
duplicate, resulting in invalid DWARF.

The DW_TAG_variant_part isn't a standalone type, so it doesn't need
an identifier. Fix by omitting its identifier.

src/test/debuginfo/enum-thinlto.rs

@@ -0,0 +1,48 @@
+// ignore-tidy-linelength
+
+// Require LLVM with DW_TAG_variant_part and a gdb that can read it.
+// min-system-llvm-version: 7.0
+// min-gdb-version: 8.2
+
+// compile-flags:-g -Z thinlto
+
+// === GDB TESTS ===================================================================================
+
+// gdb-command:run
+
+// gdb-command:print *abc
+// gdbr-check:$1 = enum_thinlto::ABC::TheA{x: 0, y: 8970181431921507452}
+
+// === LLDB TESTS ==================================================================================
+
+// lldb-command:run
+
+// lldb-command:print *abc
+// lldbg-check:(enum_thinlto::ABC) $0 = ABC { }
+
+#![allow(unused_variables)]
+#![feature(omit_gdb_pretty_printer_section)]
+#![omit_gdb_pretty_printer_section]
+
+// The first element is to ensure proper alignment, irrespective of the machines word size. Since
+// the size of the discriminant value is machine dependent, this has be taken into account when
+// datatype layout should be predictable as in this case.
+#[derive(Debug)]
+enum ABC {
+    TheA { x: i64, y: i64 },
+    TheB (i64, i32, i32),
+}
+
+fn main() {
+    let abc = ABC::TheA { x: 0, y: 0x7c7c_7c7c_7c7c_7c7c };
+
+    f(&abc);
+}
+
+fn f(abc: &ABC) {
+    zzz(); // #break
+
+    println!("{:?}", abc);
+}
+
+fn zzz() {()}