MSVC supports multiple __declspec specifiers

This isn't obvious from Microsoft's grammar, but does appear in header
files shipped with Visual Studio.

Author: tautschnig

regression/ansi-c/VS_extensions1/main.c

@@ -20,6 +20,11 @@ __declspec(thread) int thread_local;
 
 struct __declspec(dllimport) some_struct_tag { int x; };
 
+typedef union __declspec(intrin_type) __declspec(align(8)) u {
+  int x;
+  int y;
+} u2;
+
 //__delegate int GetDayOfWeek();
 
 // __event

src/ansi-c/parser.y

@@ -1440,7 +1440,20 @@ msc_declspec_opt:
         {
           init($$, ID_nil);
         }
-        | msc_declspec
+        | msc_declspec_opt msc_declspec
+        {
+          if(parser_stack($1).is_not_nil())
+          {
+            $$ = $1;
+            exprt::operandst &operands = parser_stack($1).operands();
+            operands.insert(
+              operands.end(),
+              parser_stack($2).operands().begin(),
+              parser_stack($2).operands().end());
+          }
+          else
+            $$ = $2;
+        }
         ;
 
 storage_class: