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.

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