Fix parsing for generic constraints with the "as" keyword

For generic constraints, the grammar (incorrectly) did not allow for
generic types on the right hand side of the "as" keyword. This diff fixes
the grammar appropriately.

hphp/test/quick/hh_type_args4.php

@@ -0,0 +1,12 @@
+<?hh
+class A<T> {
+  public function getT(T $x): T {
+    return $x;
+  }
+}
+class B<T> extends A<T> { }
+function foo<T, Q as A<T> >(Q $thing, T $item): T {
+  return $thing->getT($item);
+}
+echo "Done\n";
+

hphp/test/quick/hh_type_args4.php.expect

@@ -0,0 +1 @@
+Done

hphp/util/parser/hphp.y

@@ -2389,18 +2389,12 @@ hh_opt_return_type:
 ;
 
 hh_typevar_list:
-    namespace_string ','
-    hh_typevar_list                    { _p->addTypeVar($1.text()); }
- |  namespace_string                   { _p->addTypeVar($1.text()); }
- |  namespace_string T_AS
-    hh_shape_type hh_typevar_list
- |  namespace_string T_AS
-    namespace_string ','
-    hh_typevar_list                    { _p->addTypeVar($1.text()); }
- |  namespace_string T_AS
-    namespace_string                   { _p->addTypeVar($1.text()); }
- |  namespace_string T_AS
-    hh_shape_type
+    hh_typevar_list ','
+    ident                              { _p->addTypeVar($3.text()); }
+ |  ident                              { _p->addTypeVar($1.text()); }
+ |  hh_typevar_list ','
+    ident T_AS hh_type                 { _p->addTypeVar($3.text()); }
+ |  ident T_AS hh_type                 { _p->addTypeVar($1.text()); }
 ;
 
 hh_shape_member_type:

hphp/util/parser/lex.yy.cpp

@@ -77851,7 +77851,7 @@ static yyconst yy_state_type yy_NUL_trans[2434] =
 #define YY_RESTORE_YY_MORE_OFFSET
 #line 1 "hphp.ll"
 #line 2 "hphp.ll"
-#include <util/parser/scanner.h>
+#include "hphp/util/parser/scanner.h"
 
 // macros for flex
 #define YYSTYPE HPHP::ScannerToken