Improved handling and diagnostics for the `auto` storage class (#99)

Previously, CompCert would just ignore the `auto` keyword, thus accepting
incorrect top-level definitions such as
```
auto int x;
auto void f(auto int x) { }
```
This commit introduces `auto` as a proper storage class
(Storage_auto constructor in the C AST).

It adds diagnostics for misuses of `auto`, often patterned after the
existing diagnostics for misuses of `register`.

Some error messages were corrected ("storage-class" -> "storage class")
or made closer to those of clang.

Finally, in the generated C AST and in C typing environments,
block-scoped variables without an explicit storage class are recorded
as Storage_auto instead of Storage_default.  This is semantically correct
(block-scoped variables default to `auto` behavior) and will help us
distinguishing block-scoped variables from file-scoped variables
in later developments.

1 files changed, 1 insertions, 0 deletions

diff --git a/cparser/Cprint.ml b/cparser/Cprint.ml
index d623ab96..c2253c8f 100644
--- a/cparser/Cprint.ml
+++ b/cparser/Cprint.ml
@@ -361,6 +361,7 @@ let storage pp = function
   | Storage_default -> ()
   | Storage_extern -> fprintf pp "extern "
   | Storage_static -> fprintf pp "static "
+  | Storage_auto -> ()   (* used only in blocks, where it can be omitted *)
   | Storage_register -> fprintf pp "register "
 
 let full_decl pp (sto, id, ty, int) =