clightgen: handle empty names given to padding bit fields

In the Clight AST, padding bit fields (such as `int : 6;`) in composite
declarations are given an ident that corresponds to the empty string.

Previously, clightgen would give name `_` to this ident, but this is
not valid Coq.

This commit gives name `empty_ident` to the empty ident.  This name
does not start with an underscore, so it cannot conflict with the
names for regular idents, which all start with `_`.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/clightgen/bitfields.c b/test/clightgen/bitfields.c
new file mode 100644
index 00000000..34f6a686
--- /dev/null
+++ b/test/clightgen/bitfields.c
@@ -0,0 +1,13 @@
+struct s {
+  int a: 10;
+  char : 6;
+  _Bool b : 1;
+  int : 0;
+  short c: 7;
+};
+
+int f(void)
+{
+  struct s x = { -1, 1, 2 };
+  return x.a + x.b + x.c;
+}