Support Visual Studio's __forceinline

It was previously treated the same as inline/__inline, but Windows header files
come with multiple implementations of functions defined in the same file, with
an expected behaviour similar to "extern inline" in GCC.

regression/ansi-c/forceinline1/main.i

@@ -0,0 +1,15 @@
+#line 1 "test.c"
+
+__inline int foo()
+{
+  return 0;
+}
+
+__forceinline int foo()
+{
+  return 1;
+}
+
+int main()
+{
+}

regression/ansi-c/forceinline1/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.i
+--i386-win32
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/c_typecheck_base.cpp

@@ -333,10 +333,12 @@ void c_typecheck_baset::typecheck_redefinition_non_type(
         // gcc allows re-definition if the first
         // definition is marked as "extern inline"
 
-        if(old_symbol.type.get_bool(ID_C_inlined) &&
-           (config.ansi_c.mode==configt::ansi_ct::flavourt::GCC ||
-            config.ansi_c.mode==configt::ansi_ct::flavourt::APPLE ||
-            config.ansi_c.mode==configt::ansi_ct::flavourt::ARM))
+        if(
+          old_symbol.type.get_bool(ID_C_inlined) &&
+          (config.ansi_c.mode == configt::ansi_ct::flavourt::GCC ||
+           config.ansi_c.mode == configt::ansi_ct::flavourt::APPLE ||
+           config.ansi_c.mode == configt::ansi_ct::flavourt::ARM ||
+           config.ansi_c.mode == configt::ansi_ct::flavourt::VISUAL_STUDIO))
         {
           // overwrite "extern inline" properties
           old_symbol.is_extern=new_symbol.is_extern;

src/ansi-c/parser.y

@@ -165,6 +165,7 @@ extern char *yyansi_ctext;
 %token TOK_MSC_EXCEPT  "__except"
 %token TOK_MSC_LEAVE   "__leave"
 %token TOK_MSC_DECLSPEC "__declspec"
+%token TOK_MSC_FORCEINLINE "__forceinline"
 %token TOK_INTERFACE   "__interface"
 %token TOK_CDECL       "__cdecl"
 %token TOK_STDCALL     "__stdcall"
@@ -1385,6 +1386,20 @@ storage_class:
         | TOK_THREAD_LOCAL { $$=$1; set($$, ID_thread_local); }
         | TOK_GCC_ASM      { $$=$1; set($$, ID_asm); }
         | msc_declspec     { $$=$1; }
+        | TOK_MSC_FORCEINLINE
+        {
+          // equivalent to always_inline, and seemingly also has the semantics
+          // of extern inline in that multiple definitions can be provided in
+          // the same translation unit
+          init($$);
+          set($$, ID_static);
+          set($1, ID_inline);
+          $1=merge($1, $$);
+
+          init($$);
+          set($$, ID_always_inline);
+          $$=merge($1, $$);
+        }
         ;
 
 basic_type_name:

src/ansi-c/scanner.l

@@ -1138,7 +1138,7 @@ __decltype          { if(PARSER.cpp98 &&
 
 "__forceinline" { if(PARSER.mode==configt::ansi_ct::flavourt::VISUAL_STUDIO ||
                      PARSER.mode==configt::ansi_ct::flavourt::ARM)
-                    { loc(); return TOK_INLINE; }
+                    { loc(); return TOK_MSC_FORCEINLINE; }
                   else
                     return make_identifier();
                 }