Checking for clashes between field names and generated field names.

This happens when the schema is parsed, to avoid compile time errors
later, which would be harder to understand.

Bug: 16325216
Change-Id: I24cabf1adaf1700796b91e3a9641bca43a68bfbd
Tested: on OS X.

tests/test.cpp

@@ -488,6 +488,7 @@ void ErrorTest() {
   TestError("struct X { Y:int; } root_type X;", "a table");
   TestError("union X { Y }", "referenced");
   TestError("union Z { X } struct X { Y:int; }", "only tables");
+  TestError("table X { Y:[int]; YLength:int; }", "clash");
 }
 
 // Additional parser testing not covered elsewhere.