gcc: Add support for -Wmissing-prototypes in C++

Support for warnings about missing prototypes in C++ was added by Apple
GCC (Radar 6261539).  Most of the code crept into r260311 so it felt
natural to make use of it.

Obtained from:	Apple GCC - 5646
MFC after:	5 days

contrib/gcc/c.opt

@@ -280,9 +280,11 @@ Wmissing-include-dirs
 C ObjC C++ ObjC++
 Warn about user-specified include directories that do not exist
 
+; APPLE LOCAL begin warn missing prototype 6261539
 Wmissing-prototypes
-C ObjC Var(warn_missing_prototypes)
+C ObjC C++ ObjC++ Var(warn_missing_prototypes)
 Warn about global functions without prototypes
+; APPLE LOCAL end warn missing prototype 6261539
 
 ; APPLE LOCAL begin -Wmost
 Wmost

contrib/gcc/cp/decl.c

@@ -11486,6 +11486,10 @@ start_function (cp_decl_specifier_seq *declspecs,
     gcc_assert (same_type_p (TREE_TYPE (TREE_TYPE (decl1)),
 			     integer_type_node));
 
+  /* APPLE LOCAL begin warn missing prototype 6261539 */
+  check_missing_prototype (decl1);
+  /* APPLE LOCAL end warn missing prototype 6261539 */
+
   start_preparsed_function (decl1, attrs, /*flags=*/SF_DEFAULT);
 
   return 1;

contrib/gcc/doc/invoke.texi

@@ -225,6 +225,8 @@ in the following sections.
 -Wmain  -Wmissing-braces  -Wmissing-field-initializers @gol
 -Wmissing-format-attribute  -Wmissing-include-dirs @gol
 -Wmissing-noreturn @gol
+@c APPLE LOCAL warn missing prototype 6261539
+-Wmissing-prototypes @gol
 @c APPLE LOCAL -Wmost
 -Wmost (APPLE ONLY) @gol
 -Wno-multichar  -Wnonnull  -Wno-overflow @gol
@@ -245,7 +247,8 @@ in the following sections.
 
 @item C-only Warning Options
 @gccoptlist{-Wbad-function-cast  -Wmissing-declarations @gol
--Wmissing-prototypes  -Wnested-externs  -Wold-style-definition @gol
+@c APPLE LOCAL warn missing prototype 6261539
+-Wnested-externs  -Wold-style-definition @gol
 -Wstrict-prototypes  -Wtraditional @gol
 -Wdeclaration-after-statement -Wpointer-sign}
 
@@ -3052,7 +3055,8 @@ types.)
 Warn if an old-style function definition is used.  A warning is given
 even if there is a previous prototype.
 
-@item -Wmissing-prototypes @r{(C only)}
+@c APPLE LOCAL warn missing prototype 6261539
+@item -Wmissing-prototypes
 @opindex Wmissing-prototypes
 Warn if a global function is defined without a previous prototype
 declaration.  This warning is issued even if the definition itself