Support GCC's fallthrough attribute

regression/ansi-c/gcc_attributes13/main.c

@@ -0,0 +1,20 @@
+// GCC statement attributes -- currently "fallthrough" is the only one that GCC
+// supports.
+// https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html
+
+int main()
+{
+  int x;
+  switch(x)
+  {
+    case 1:
+      x = 2;
+#ifdef __GNUC__
+      __attribute__((fallthrough));
+#endif
+    case 2:
+      x = 3;
+  }
+
+  return 0;
+}

regression/ansi-c/gcc_attributes13/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/parser.y

@@ -148,6 +148,7 @@ extern char *yyansi_ctext;
 %token TOK_GCC_ATTRIBUTE_NORETURN "noreturn"
 %token TOK_GCC_ATTRIBUTE_CONSTRUCTOR "constructor"
 %token TOK_GCC_ATTRIBUTE_DESTRUCTOR "destructor"
+%token TOK_GCC_ATTRIBUTE_FALLTHROUGH "fallthrough"
 %token TOK_GCC_LABEL   "__label__"
 %token TOK_MSC_ASM     "__asm"
 %token TOK_MSC_BASED   "__based"
@@ -2156,6 +2157,7 @@ statement:
         | msc_asm_statement
         | msc_seh_statement
         | cprover_exception_statement
+        | statement_attribute
         ;
 
 declaration_statement:
@@ -2214,6 +2216,14 @@ labeled_statement:
         }
         ;
 
+statement_attribute:
+          TOK_GCC_ATTRIBUTE '(' '(' TOK_GCC_ATTRIBUTE_FALLTHROUGH ')' ')' ';' labeled_statement
+        {
+          // attribute ignored
+          $$=$8;
+        }
+        ;
+
 compound_statement:
           compound_scope '{' '}'
         {

src/ansi-c/scanner.l

@@ -1580,6 +1580,8 @@ __decltype          { if(PARSER.cpp98 &&
 "destructor" |
 "__destructor__"   { BEGIN(GCC_ATTRIBUTE3); loc(); return TOK_GCC_ATTRIBUTE_DESTRUCTOR; }
 
+"fallthrough"       { BEGIN(GCC_ATTRIBUTE3); loc(); return TOK_GCC_ATTRIBUTE_FALLTHROUGH; }
+
 {ws}                { /* ignore */ }
 {newline}           { /* ignore */ }
 {identifier}        { BEGIN(GCC_ATTRIBUTE4); }