Add enum code generation

This adds a naieve first pass approach to enum type code generation. The
original idea was to use GCC's QUAL_UNION_TYPE but I have ran into issues
with the DECL_QUALIFIER as my understanding of how this works is incorrect.

This takes an enum such as:

```rust
enum AnEnum {
  A,
  B,
  C (char),
  D (x: i64, y: i64),
}
```

And turns this into one big union consisting of all fields as RECORD_TYPES.

```c
union AnEnum {
  record A { RUST$ENUM$DISR };
  record B { RUST$ENUM$DISR };
  record C { RUST$ENUM$DISR, char };
  record D { RUST$ENUM$DISR, i64, i64};
}
```

see: https://github.com/bminor/binutils-gdb/blob/527b8861cd472385fa9160a91dd6d65a25c41987/gdb/dwarf2/read.c#L9010-L9241

With the RUST$ENUM$DISR being the first field in all of the records this
means the alignment allows for indirect memory access of the struct to
use it as a qualifier field to figure out which variant is currently in
use. The data-less varients use their generated discriminat value during
type-checking the data variants use their HIR ID for their discriminant.

This will likely get redone to get improved GDB integration/updated to use
the QUAL_UNION_TYPE when we learn how to do this properly.

Fixes #79

1 files changed, 3 insertions, 1 deletions

diff --git a/gcc/rust/rust-backend.h b/gcc/rust/rust-backend.h
index 64144bc..43a81f2 100644
--- a/gcc/rust/rust-backend.h
+++ b/gcc/rust/rust-backend.h
@@ -122,6 +122,8 @@ public:
     return "unknown";
   }
 
+  virtual tree get_identifier_node (const std::string &str) = 0;
+
   // Types.
 
   // Produce an error type.  Actually the backend could probably just
@@ -348,7 +350,7 @@ public:
   // Return an expression that constructs BTYPE with VALS.  BTYPE must be the
   // backend representation a of struct.  VALS must be in the same order as the
   // corresponding fields in BTYPE.
-  virtual tree constructor_expression (tree btype,
+  virtual tree constructor_expression (tree btype, bool is_variant,
 				       const std::vector<tree> &vals, int,
 				       Location)
     = 0;