Never allow typedefs with the same names as classes

Right now, we allowed it as long as the typedef was for the
same class.  (Oversight.)  This involved a few error message changes
in existing tests.

hphp/runtime/vm/unit.cpp

@@ -684,7 +684,8 @@ void Unit::defTypedef(Id id) {
 
   // There might also be a class with this name already.
   if (Class* cls = nameList->getCachedClass()) {
-    checkExistingClass(cls);
+    raise_error("The name %s is already defined as a class",
+                thisType->m_name->data());
     return;
   }
 

hphp/test/quick/typedef_duplicate3.php.expectf

@@ -1 +1 @@
-HipHop Fatal error: The type Existing is already defined to a different class (Existing) in %s on line 6
+HipHop Fatal error: The name Existing is already defined as a class in %s on line 6

hphp/test/quick/typedef_duplicate4.php.expectf

@@ -1 +1 @@
-HipHop Fatal error: The type Something is already defined to a different class (Something) in %s on line 4
+HipHop Fatal error: The name Something is already defined as a class in %s on line 4

hphp/test/quick/typedef_unique.php

@@ -0,0 +1,8 @@
+<?hh
+
+class Foo {
+}
+
+type Foo = Foo;
+
+function main() {}

hphp/test/quick/typedef_unique.php.expectf

@@ -0,0 +1 @@
+HipHop Fatal error: The name Foo is already defined as a class in %s on line 8